Biological effects of ionizing radiation in medical imaging
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DOCTORAL DISSERTATION
Biological effects of ionizing radiation in medical imaging: a prospective study in children and adults following dental cone-beam computed tomography
Doctoral dissertation submitted to obtain the degree of Doctor of Biomedical Sciences, to be defended by
Promoter: Prof. Dr Ivo Lambrichts | UHasselt Co-promoters: Prof. Dr Stéphane Lucas | Université de Namur Dr Marjan Moreels | SCK-CEN
2019 | Faculty of Medicine and Life Sciences
D/2019/2451/55
Niels Belmans
Table of contents
III
Table of contents ................................................................................... I
Table of tables .....................................................................................IX
Table of figures ................................................................................. XIII
List of abbreviations ......................................................................... XVII
Introduction.......................................................................................... 1
1.1 Ionizing radiation ........................................................................ 3
1.1.1 What is ionizing radiation? ..................................................... 3
1.1.2 Radiation doses and units ...................................................... 3
1.2 The use of X-rays in medical diagnostics ........................................ 7
1.2.1 Introduction to medical radiation exposure .............................. 7
1.2.2 Radiography, computed tomography, and cone beam computed
tomography ........................................................................ 8
1.2.2.1 Radiography ............................................................... 8
1.2.2.2 Computed tomography ................................................ 9
1.2.2.3 Cone beam computed tomography ................................ 9
1.2.3 Radiation protection in medical imaging ................................ 10
1.2.4 Health risks associated with medical diagnostic procedures ..... 11
1.2.4.1 Epidemiological data on medical diagnostic exposure ..... 12
1.2.4.2 How to cope with limited data on health effects related to
CBCT examinations? .................................................. 12
1.3 Cellular and subcellular effects following ionizing radiation exposure 14
1.3.1 Direct and indirect effects of ionizing radiation ....................... 14
1.3.2 Oxidative stress ................................................................. 15
1.3.2.1 Generation and effect of reactive oxygen species by
ionizing radiation ...................................................... 15
1.3.2.2 Cellular defence mechanisms against oxidative stress .... 17
1.3.3 Oxidative stress measurements ........................................... 18
1.3.3.1 Oxidation of proteins ................................................. 19
1.3.3.2 Oxidation of lipids ..................................................... 20
1.3.3.3 Oxidation of DNA ...................................................... 20
1.3.3.4 Markers of antioxidant defence ................................... 20
1.3.3.5 Oxidative stress after low dose radiation exposure ........ 21
1.3.4 Radiation-induced DNA damage and the DNA damage response21
1.3.4.1 DNA damage mediators ............................................. 22
Table of contents
IV
1.3.4.2 DNA damage effectors ............................................... 23
Cell cycle checkpoints ........................................................ 23
DNA damage repair pathways ............................................. 25
Removal of severely damaged, non-functioning cells ............. 27
1.3.5 DNA damage measurements ................................................ 31
1.3.5.1 Assessing DNA damage and repair through the
γH2AX/53BP1 assay .................................................. 32
1.3.5.2 Cell cycle analysis ..................................................... 33
1.3.5.3 Premature cellular senescence .................................... 34
1.4 Radiation protection: guidelines and risk assessment ..................... 35
1.5 The oral cavity .......................................................................... 37
1.5.1 Dental stem cells ................................................................ 37
1.5.2 Buccal mucosal cells ........................................................... 39
1.5.3 Saliva ............................................................................... 39
1.6 References ............................................................................... 41
Scope and aim of the research .............................................................. 53
References ..................................................................................... 58
Method validation to assess in vivo cellular and subcellular changes in buccal
mucosa cells and saliva following CBCT examinations .............................. 63
3.1 Abstract ................................................................................... 65
3.2 Introduction .............................................................................. 66
3.3 Materials and methods ............................................................... 69
3.3.1 Description of the DIMITRA protocol ..................................... 69
3.3.2 Buccal mucosal cell collection and fixation ............................. 70
3.3.3 Immunocytological staining for DNA double strand breaks:
γH2AX and 53BP1 staining .................................................. 71
3.3.4 Saliva collection and analysis ............................................... 72
3.3.5 8-oxo-dG determination ...................................................... 73
3.3.6 Total antioxidant capacity .................................................... 73
3.4 Protocol validation ..................................................................... 75
3.4.1 Pilot study population ......................................................... 75
3.4.2 Flow cytometrical identification of buccal mucosal cells ........... 75
3.4.3 Histological staining for epithelial cell identification ................. 76
3.4.4 Statistics ........................................................................... 76
3.5 Results ..................................................................................... 77
3.6 Discussion ................................................................................ 80
Table of contents
V
3.7 Conclusion ................................................................................ 83
3.8 References ............................................................................... 84
Dental cone beam CT examination induces oxidative damage and antioxidant
response in children’s saliva ................................................................. 89
4.1 Abstract ................................................................................... 91
4.2 Uncertainties concerning low dose ionizing radiation exposure and
medical imaging ....................................................................... 92
4.3 Materials & Methods................................................................... 95
4.3.1 EU OPERRA - DIMITRA study ............................................... 95
4.3.2 Patient selection ................................................................. 95
4.3.3 Buccal mucosal cell collection and immunocytological staining . 95
4.3.4 Saliva collection ................................................................. 96
4.3.5 8-oxo-dG enzyme-linked immunosorbent assay ..................... 97
4.3.6 Total antioxidant capacity determination ............................... 97
4.3.7 Dose calculations – Monte Carlo simulation ............................ 97
4.3.8 Statistics ........................................................................... 98
4.4 Results ..................................................................................... 99
4.4.1 Patients and dose exposure ................................................. 99
4.4.2 DNA double strand break detection in exfoliated buccal mucosal
cells before and after CBCT examination ............................... 99
4.4.3 8-oxo-dG levels in saliva samples ....................................... 100
4.4.4 Total antioxidant capacity in saliva samples ......................... 103
4.5 Discussion .............................................................................. 105
4.6 Competing interests ................................................................. 110
4.7 Acknowledgements .................................................................. 110
4.8 References ............................................................................. 111
4.8 Supplementary Data ................................................................ 116
4.8.1 Supplementary Data 1 ...................................................... 116
4.8.2 Supplementary Data 2 ...................................................... 117
4.8.3 Supplementary Data 3 ...................................................... 118
4.8.4 Supplementary Data 4 ...................................................... 119
4.8.5 Supplementary Data 5 ...................................................... 120
4.8.6 Supplementary Table 1 ..................................................... 121
In vitro assessment of the DNA damage response in dental stem cells
following low dose X-ray exposure ....................................................... 125
5.1 Abstract ................................................................................. 127
Table of contents
VI
5.2 Introduction ............................................................................ 128
5.3 Material and methods .............................................................. 131
5.3.1 Culturing dental stem cells ................................................ 131
5.3.2 X-irradiation conditions ..................................................... 131
5.3.4 Immunocytochemical staining for γH2AX and 53BP1 ............. 132
5.3.7 Cell cycle analysis ............................................................ 133
5.3.8 Quiescence assay ............................................................. 133
5.3.9 Β-galactosidase assay ....................................................... 133
5.3.10 Enzyme-linked immunosorbent assay (ELISA): IL-6, IL-8,
IGFBP-2, and IGFBP-3 ...................................................... 134
5.3.11 Statistical analysis .......................................................... 134
5.4 Results ................................................................................... 135
5.4.1 Exposure to low doses of X-rays induces DSBs and activates the
DNA damage response in dental stem cells ......................... 135
5.4.2 Cell cycle progression is not influenced by low doses of X-rays in
dental stem cells .............................................................. 137
5.4.3 Low dose X-irradiation rapidly decreases the amount of quiescent
cells ............................................................................... 138
5.4.4 Low dose radiation does not induce premature senescence in
dental stem cells .............................................................. 139
5.5 Discussion .............................................................................. 142
5.6 References ............................................................................. 145
Antioxidant response in buccal mucosa cells and saliva samples following
CBCT examination ............................................................................. 149
6.1 Introduction ............................................................................ 151
6.2 Materials and methods ............................................................. 153
6.2.1 Patient selection ............................................................... 153
6.2.2 Saliva collection ............................................................... 153
6.2.3 Buccal mucosa cell collection ............................................. 153
6.2.4 Enzyme activity assay ....................................................... 153
6.2.5 RNA isolation from RNAprotect Cell Reagent ........................ 154
6.2.6 cDNA synthesis ................................................................ 154
6.2.7 Gene expression analysis using TaqManTM probes and primers 155
6.2.8 Dose calculations – Monte Carlo simulation .......................... 155
6.2.9 Statistical analysis ............................................................ 156
Table of contents
VII
6.3 Results ................................................................................... 157
6.3.1 Patients and dose exposure ............................................... 157
6.3.1 CBCT examination leads to an increase in SOD activity which is
dependent on gender ....................................................... 157
6.3.2 CBCT examination leads to an increase in CAT activity .......... 158
6.3.3 Changes in SOD1, CAT, and GPx1 gene expression in children
and adults ....................................................................... 160
6.4 Discussion .............................................................................. 162
6.5 References ............................................................................. 164
General discussion and future perspectives ........................................... 167
7.1 General discussion ................................................................... 169
7.2 Future perspectives ................................................................. 177
7.3 References ............................................................................. 183
Summary ......................................................................................... 191
Samenvatting ................................................................................... 195
Appendices ....................................................................................... 199
Appendix 1: Overview of the biological effects detected in patients following
computed tomography ............................................................ 201
Appendix 2: Overview of the biological effects detected in patients following
X-ray radiography .................................................................. 209
Appendix 3: Overview of the biological effects detected in patients following
cone beam computed tomography ............................................ 215
Curriculum Vitae ............................................................................... 217
List of publications ............................................................................. 223
Acknowledgements ............................................................................ 229
Table of tables
XI
Table 1.1. Overview of different radiation dose units ...................................... 5
Table 1.2. ICRP recommended radiation weighting factors .............................. 6
Table 1.3. ICRP recommended tissue weighting factors .................................. 6
Table 3.1. Overview of scan parameters per patient included in this validation
study. ..................................................................................... 77
Table 4.1. Comparison between boys and girls for 8-oxo-dG excretion before and
after cone beam computed tomography (CBCT) examination. ...... 102
Table 4.2. Comparison between boys and girls FRAP values before and after cone
beam computed tomography (CBCT) examination. ..................... 104
Supplementary table 1. Individual patient study parameters of included patients.
............................................................................................ 121
Table 5.1: Overview of dental stem cell donors.......................................... 131
Table 5.2: Linear dose response relationship of co-localized γH2AX and 53BP1 foci
in dental stem cells ................................................................. 137
Table 5.3: Significant differences in the percentage of quiescent cells in dental
stem cells .............................................................................. 139
Table 6.1: Overview of patients included in this study up to now ................. 157
Table of figures
XV
Figure 1.1. Overview of the frequency of medical diagnostic procedures per 1000
capita in the European Union (top panel) and of the effective dose per
caput (bottom panel). ................................................................. 8
Figure 1.2. Comparison of oral radiograph (A.), oral cone-beam computed
tomography (B.) and oral computed tomography (C.) images. ....... 10
Figure 1.3. Biological effects of ionizing radiation ......................................... 14
Figure 1.4. Overview of the direct and indirect actions of ionizing radiation. .... 16
Figure 1.5. Generation and metabolism of reactive oxygen species by enzymatic
antioxidants. ............................................................................ 18
Figure 1.6. General overview of the DNA damage response. .......................... 22
Figure 1.7. Error-free homologous recombination (HR) compared to error-prone
non-homologous end joining (NHEJ). .......................................... 26
Figure 1.8. Overview of the four main modes of removing non-functioning cells
induced by DNA damage. .......................................................... 27
Figure 1.9. Overview of molecular pathways involved in damage-induced
senescence. ............................................................................. 29
Figure 1.10. Extrinsic and intrinsic apoptotic pathways. ................................ 31
Figure 1.11. Overview of the cell cycle ....................................................... 34
Figure 1.12. Graphical representation of the different models explaining the dose-
response relationship in the low dose range. ................................ 36
Figure 1.13. Overview of the anatomy of the oral cavity. .............................. 37
Figure 1.14. Overview of the different types of dental stem cells and their in vivo
location. .................................................................................. 38
Figure 3.1. Flow chart for patient inclusion and patient sampling. .................. 70
Figure 3.2. Flow chart for sample analysis .................................................. 74
Figure 3.3. Flow cytometrical identification of cells collected by buccal swab ... 78
Figure 3.4. Microscopical identification of cells collected by buccal swab ......... 79
Figure 4.1. No DNA double strand breaks (DSBs) are induced in buccal mucosal
cells (BMCs) after cone beam computed tomography (CBCT)
examination, neither in children nor in adults ............................. 100
Figure 4.2. Excretion of 8-oxo-7,8-dihydro-2’-deoxyguanosine (8-oxo-dG) into
saliva is increased after cone beam computed tomography (CBCT)
examination in children but not in adults. .................................. 101
Figure 4.3. No dose response in 8-oxo-dG excretion in saliva 30 minutes after
cone beam computed tomography in children. ........................... 102
Figure 4.4. Ferric reducing antioxidant power (FRAP) values increase in saliva
samples from children after cone beam computed tomography (CBCT)
examination, while decreasing in saliva samples from adults. ....... 103
Figure 5.1. DNA double strand break formation and repair kinetics. ............. 136
Figure 5.2. Cell cycle analysis of dental pulp stem cells from deciduous teeth.138
Figure 5.3. Dose response of the percentage of G0 phase dental pulp stem cells
from deciduous teeth and stem cells from the apical papilla following
low dose X-irradiation. ............................................................ 139
Table of figures
XVI
Figure 5.4. Senescence-associated secretory phenotype (SASP) protein secretion
in dental pulp stem cells fro ..................................................... 140
Figure 5.5. β-galactosidase assay in dental stem cells. ............................... 141
Figure 6.1. Superoxide dismutase (SOD) activity 30 minutes after CBCT
examination in saliva samples from children .............................. 158
Figure 6.2. Gender differences in superoxide dismutase (SOD) activity after CBCT
examination in saliva samples from children .............................. 158
Figure 6.3. Catalase (CAT) activity 30 minutes after CBCT examination in saliva
samples from children ............................................................. 159
Figure 6.4. Catalase (CAT) activity 30 minutes after CBCT examination in saliva
samples from boys and girls .................................................... 160
Figure 6.5. Relative gene expression changes in the SOD1, CAT, and GPx1 genes
in children and adults .............................................................. 161
List of abbreviations
XIX
2D Two-dimensional
3D Three-dimensional
4-HNE trans-4-hydroxy-2-
nonenal
53BP1 p53-binding protein 1
7-AAD 7-aminoactinomycin D
8-oxo-dG 7,8-dihydro-8-oxo-2’-
deoxyguanosine
α Alpha radiation
β Beta radiation
γ Gamma radiation
γH2AX Phosphorylated histone
2AX on Serine 139
ωR Radiation weighting
factor
ωT Tissue-weighting factor
AGEs Advanced glycation end
products
ALARA As-low-as-reasonable-
achievable
ANOVA Analysis of variance
ATM Ataxia telangiectasia
mutated
ATR Ataxia telangiectasia
and Rad3-related
protein
BER Base excision repair
BM Buccal mucosa
BMCs Buccal mucosa cells
BRCA1 Breast cancer early
onset 1
BRCT Breast cancer 1 C-
terminal
BrdU 5-bromo-2’-
deoxyuridine
BuBu Buccal buffer
CAT Catalase
CBCT Cone-beam computed
tomography
CDK Cyclin-dependent
kinase
Chk Checkpoint kinase
CT Computed tomography
CTDIvol Volume computed
tomography dose index
CVD Cardiovascular disease
DAPI 4’-6-diamidino-2-
phenylindole
DDR DNA damage response
DF Degrees of freedom
DFSCs Dental follicle stem cells
DIMITRA Dentomaxillofacial
Paediatric Imaging: An
Investigation Towards
Low Dose Radiation
Induced Risks
DLP Dose-length product
DNA Deoxyribonucleic acid
DNA-PK DNA-dependent protein
kinase
DPSCs Dental pulp stem cells
DSBs Double strand breaks
ED Effective dose
ELISA Enzyme-linked
immunosorbent assay
ESR Election spin resonance
FOV Field of view
FRAP Ferric reducing
antioxidant power
FWO Fonds Wetenschappelijk
Onderzoek Vlaanderen
GSH Reduced glutathione
GSH-Px Glutathione peroxidase
GSSG Glutathione disulphide
Gy Gray
H2O2 Hydrogen peroxide
HLEG European High-Level
Expert Group on
European Low Dose
Risk Research
HPLC High-performance liquid
chromatography
HR Homologous
recombination
List of abbreviations
XX
ICRP International
Commission on
Radiological Protection
IGF Insulin-like growth
factor
IGFBP Insulin-like growth
factor binding protein
IL Interleukin
IR Ionizing radiation
IRIF Ionizing radiation-
induced foci
kV Tube voltage
LC-MS Liquid chromatography
tandem mass
spectrometry
LNT Linear-no-threshold
mAs Tube current-exposure
time product
MC Monte Carlo
MDA Malonaldehyde
MDC1 Mediator of the DNA
damage checkpoint 1
miRNA Micro-RNA
MMR Mismatch repair
MN Micronucleus
MRN Mre11/Rad50/NBs1
complex
MSCs Mesenchymal stem cells
NER Nucleotide excision
repair
NF-κβ Nuclear factor κβ
NHEJ Non-homologous end-
joining
O2- Superoxide anion
OECD Organisation for
Economic Cooperation
and Development
OH• Hydroxyl radical
OPERRA Open Project for
European Radiation
Research Area
oxLDL Oxidized low-density
lipoproteins
PBS Phosphate-buffered
saline
PCR Polymerase chain
reaction
PDLSCs Periodontal ligament
stem cells
PFA Paraformaldehyde
PIG Pre-immunized goat
serum
PIKK Phosphatidylinositol 3-
kinase like kinase
qPCR Real-time polymerase
chain reaction
REID Risk of exposure-
induced death
ROS Reactive oxygen
species
RT Room temperature
SASP Senescence-associated
secretory phenotype
SCAPs Stem cells from the
apical papilla
SCK•CEN Belgian Nuclear
Research Centre
SEM Standard error of the
mean
SF Saccomanno’s fixative
SHEDs Stem cells from human
exfoliated deciduous
teeth
SI International System of
the Units
SOD Superoxide dismutase
SSBs Single strand breaks
Sv Sievert
TGFβ Transforming growth
factor β
Introduction
3
1.1 Ionizing radiation
1.1.1 What is ionizing radiation?
On earth people are continuously exposed to ionizing radiation (IR) from
natural and/or man-made sources. IR can be defined as electromagnetic waves
or particles that have enough energy to eject electrons from their orbit in an atom.
This ejection causes the atom to become ionized, hence ‘ionizing radiation’.(1, 2)
IR originates from natural processes and from man-made sources. In nature,
three main types of IR can be identified: alpha (α), beta (β) and gamma (γ)
radiation. They occur naturally when unstable nuclei (e.g. cobalt-60) undergo
spontaneous radioactive decay. During radioactive decay, unstable nuclei will emit
energy in the form of IR, i.e. electromagnetic particles (α and/or β) or waves (γ),
in order to become energetically more stable.(3) X-rays are the most common form
of man-made IR. They are mostly identical to γ-radiation but their origin is
different: γ-radiation comes from the natural process of radioactive decay,
whereas X-rays are produced by man-made X-ray generators. Man-made IR
sources include 1) the use of IR in medicine, such as X-rays in diagnostics and X-
rays, protons and carbon ions in radiotherapy treatment, and nuclear medicine
procedures using radioactive isotopes, 2) radiation exposure from the nuclear fuel
cycle, such as uranium, which decays by emitting α, β, and γ radiation, and 3)
exposure to radioactive fallout from nuclear weapons/accidents, such as the
atomic bombs in Hiroshima and Nagasaki (1945) and the nuclear reactor accidents
in Chernobyl (1986) and Fukushima (2011).(4)
α particles are subatomic particles consisting of two protons and two
neutrons, which is in fact a helium core. They are relatively heavy, positively
charged and energy-rich particles. β particles can be either negatively charged
electrons or positively charged positrons. Their energy is intermediate between α
particles and γ-rays/X-rays. γ-rays and X-rays are massless, electrically neutral,
packets of energy, also known as photons.(2) Finally, besides the main types of IR
discussed here, other types of natural and man-made IR exist (e.g. neutrons and
accelerated ions).(5, 6)
1.1.2 Radiation doses and units
IR is ubiquitous and can have a major impact on human health. Therefore,
it is important to know which energy or which radiation dose is absorbed by the
human body and its organs. Furthermore, in order to study radiation-induced
health effects, it is important to know the excessive risk associated with a certain
radiation dose. To this end, several units are used in the International System of
Introduction
4
Units (SI) to express radiation doses: the absorbed dose, the equivalent dose and
the effective dose. Besides these three widely used dose units, others are used,
which are related to specific fields. In medical diagnostics for example, the volume
computed tomography dose index (CTDIvol) and the dose-length product (DLP)
are used alongside the previously mentioned ones (Table 1.1).(7-9)
The absorbed dose represents the amount of radiation energy that is
absorbed per unit of mass of a substance. The SI unit is Gray (Gy). The absorbed
dose does not take into account the radiation type nor its biological effect on
tissues and organs.(10) The equivalent dose takes into account the radiation type
and its effectiveness. It is calculated as the absorbed dose multiplied by a radiation
weighting factor (ωR), which is an estimate of the effectiveness per dose unit of a
given radiation type compared to a standard (Table 1.2). The SI unit is Sievert
(Sv).(7, 8) The effective dose is defined as the weighted sum of all tissue and organ
equivalent doses multiplied by their respective tissue-weighting factor (𝜔𝑇), which
is a relative measure of the risk of stochastic effects that could result from
radiation exposure of a specific tissue (Table 1.3). It represents the health risk,
i.e. the probability of carcinogenesis and/or genotoxic effects of IR. The SI unit is
Sv.(8) Both ωR and 𝜔𝑇 are recommended by the International Commission on
Radiological Protection (ICRP).(10)
Introduction
5
T
ab
le 1
.1.
Overv
iew
of
dif
feren
t rad
iati
on
dose u
nit
s.(
11
)
Wh
at
does i
t m
ean
?
Repre
sents
the a
mount
of
radia
tion e
nerg
y t
hat
is a
bsorb
ed p
er
unit o
f m
ass o
f a s
ubsta
nce.
Takes into
account
the t
ype o
f ra
dia
tion a
s w
ell
as its effectiveness.
When exposed to
m
ultip
le
radia
tion ty
pes,
the equiv
ale
nt
doses of
each
radia
tion
type
must
be
calc
ula
ted
and
then
sum
mate
d.
(7,
8)
Takes in
to account
the equiv
ale
nt
doses in
all
specifie
d t
issues a
nd o
rgans o
f th
e b
ody,
whic
h
is
multip
lied
by
a
tissue-s
pecific
w
eig
hting
facto
r.
Repre
sents
th
e
health
risk,
i.e.
the
pro
bability of
cancer
induction and/o
r genetic
effects
.(8)
Quantifies t
he r
ela
tive inte
nsity o
f th
e r
adia
tion
that
is
delivere
d
to
the
patient
during
a
com
pute
d t
om
ogra
phy e
xam
ination.(9
)
Used to
calc
ula
te th
e to
tal
absorb
ed dose of
radia
tion a
patient
is e
xposed t
o in a
com
pute
d
tom
ogra
phy
exam
ination
and
is
there
fore
directly re
late
d to
th
e sto
chastic risk.(9
) N
ote
:
DLP is n
ot
equal to
the e
ffective d
ose.
ε̅ =
mean e
nerg
y; mT =
mass o
f volu
me o
f in
tere
st;
DT,R
= D
in a
targ
et
tissue (
T)
due t
o r
adia
tion t
ype ‘R’;
ωR =
radia
tion
weig
hting facto
r; 𝜔
𝑇 =
tis
sue w
eig
hting f
acto
r; 𝑟𝑒𝑚
= r
em
ain
der
tissues
Calc
ula
tion
D =
ε̅
mT
HT =
∑
ωRDT,R
R
E =
∑𝜔𝑇𝐻𝑇
𝑇+𝜔𝑟𝑒𝑚𝐻𝑟𝑒𝑚
((1/3
) x r
adia
tion
cente
r +
(2/3
)
x r
adia
tion
periphery)/
pitch
CTD
I vol x s
can length
Sym
bol
D
HT
E
CTD
I vol
DLP
Un
it
Gra
y (
Gy)
(J•kg-1
)
Sie
vert
(Sv)
(J•kg-1
)
Sie
vert
(Sv)
(J•kg-1
)
Gra
y (
Gy)
Gra
y•centim
ete
r
(Gy•cm
)
Rad
iati
on
do
se
Absorb
ed d
ose
Equiv
ale
nt
dose
Eff
ective d
ose
Volu
me
com
pute
d
tom
ogra
phy
dose index
Dose-l
ength
pro
duct
Introduction
6
Table 1.2. ICRP recommended radiation weighting factors(10)
Radiation type 𝛚𝐑*
Photons (X-rays, gamma rays) 1
Electrons and muons 1
Protons and charged ions 2
α particles, fission fragments, heavy ions 20
Neutrons A continuous function of neutron energy
*:ωR = radiation weighting factor
ICRP = International Commission on Radiological Protection
Table 1.3. ICRP recommended tissue weighting factors(10)
Target organ 𝛚𝐓* ∑𝝎𝑻
𝑻
Red bone-marrow, colon, lung, stomach, breast, remainder tissues**
0.12 0.72
Gonads 0.08 0.08
Bladder, oesophagus, liver, thyroid 0.04 0.16
Bone surface, brain, salivary glands, skin
0.01 0.04
Total 1.00
*:𝜔𝑇 = tissue weighting factor; **: Remainder tissues include adrenals, extra-thoracic
region, gallbladder, heart, kidneys, lymphatic nodes, muscles, oral mucosa, pancreas, prostate, small intestine, spleen, thymus, uterus/cervix. ICRP = International Commission on Radiological Protection
Introduction
7
1.2 The use of X-rays in medical diagnostics
1.2.1 Introduction to medical radiation exposure
The discovery of X-rays by Sir Wilhelm Conrad Roentgen in 1895 led to a
revolution in the field of medicine. Soon after its discovery, X-ray machines were
built that made it possible to study and/or treat internal structures of the body,
without the need for dissection. Therefore, IR was increasingly used in medicine,
leading to the medical fields of radiology and radiotherapy, and today IR is an
indispensable tool for both medical diagnostics and therapy.(12) For most of the
20th century, radiography was limited to two-dimensional radiographs. This
changed in 1972 with the introduction of computed tomography (CT) by
Hounsfield. The evolutionary CT allowed for three-dimensional (3D) imaging,
which led to more accurate diagnostics and new treatment strategies.(13) The
introduction of CT further increased the use of IR in medical diagnostics and
treatment.
The use of IR in medical diagnostics (e.g. CT and two-dimensional
radiography) has increased globally from 280 per 1000 capita in 1988 to 488 per
1000 capita in 2008, an average increase of 74%. This remarkable increase in the
amount of examinations using IR coincides with an increase in the global average
annual effective dose (contributed by medical diagnostics) per caput. In 1988, the
average effective dose per caput was 0.35 mSv, whereas in 2008, the average
effective dose per caput was 0.62 mSv. An increase of 77% in IR exposure due to
medical applications (excluding radiotherapy). Currently, medical applications of
IR account for about 14% of the total annual exposure worldwide, which makes
medical applications the largest man-made source of IR exposure to the general
population.(12, 14) Data available from the European Commission show that in the
36 countries from which data are collected, radiography is by far the most
frequently used in the clinic, followed by CT, fluoroscopy, and finally interventional
radiology (Figure 1.1). Despite the high frequency of radiographs taken, the
radiation burden due to CT is the highest in almost all countries of the European
Union. It is estimated that CT examinations account for 55% of the annual
effective dose in Europe. Radiography examinations account for 23% of the annual
effective dose, followed by fluoroscopy (13%) and interventional radiology
(9%).(15) On average, the radiation doses used in CT range from 15 mSv to 30
mSv, in adults and neonates respectively.(16) In radiography, the radiation doses
range from 0.001 mSv to 0.1 mSv. In fluoroscopy examinations, the radiation
doses vary between 0.4 mSv and 5 mSv.(17) Next, in interventional radiology, the
cumulative air kerma ranges from 4 mGy to 3230 mGy at the patient entrance
reference point.(18) These data explain why the average effective radiation dose
due to CT examinations is much higher than that of radiographs, fluoroscopy and
interventional radiology.(19)
Introduction
8
In the following paragraphs, the focus will be on the use and potential health
risks of two-dimensional radiography and CT, the two most frequently used
medical imaging techniques with X-rays, as well as on the use and potential health
risks of cone-beam computed tomography (CBCT), a relatively new CT-based
imaging technique.
Figure 1.1. Overview of the frequency of medical diagnostic procedures per 1000
capita in the European Union (top panel) and of the effective dose per caput
(bottom panel). Radiography are the most used in medical diagnostics, however the
exposure due to computed tomography is by far the highest. Note that the radiation
exposure in Belgium is highest among the participating countries. Data on the effective dose
due to fluoroscopy and interventional radiology were not available for Greece (EL).(15)
1.2.2 Radiography, computed tomography, and cone beam computed
tomography
1.2.2.1 Radiography
Radiographs have been widely used in medicine since shortly after the
discovery of X-rays (Figure 1.2). Currently, radiographs are the most frequently
used diagnostic imaging modality (Figure 1.1). X-ray radiographs are mostly used
Introduction
9
for bone and dental examinations, orthopaedic evaluations, chiropractic
examinations, and mammography.(19)
The average radiations doses associated with radiographs are very low and
range from 0.001 mSv to 0.1 mSv. However, these doses could rapidly increase
when multiple radiographs have to be taken. The average effective dose related
to radiography has been steadily decreasing since the 1970s due to better X-ray
equipment and improvement of radiation protection guidelines.(12, 19)
1.2.2.2 Computed tomography
Since its introduction in the 1970s, the use of CT has increased rapidly
(Figure 1.2). In Belgium, for example, 180 examinations per 1000 capita were
performed in 2008. In 2017, this number increased to 200 examinations per 1000
capita, meaning one in five inhabitants is subjected to a CT examination per
year.(20) The Organisation for Economic Cooperation and Development (OECD)
calculated that the use of CT scans in the OECD countries ranged from 37 per
1000 capita (Finland) to 231 per 1000 capita (Japan).(21) CT scans are mostly used
for diagnosis, such as bone disorders, but CT can also be used to detect internal
bleedings, localize tumours, to guide surgeons during surgery or radiotherapy
treatment, and to monitor disease or treatment progression.(16)
As mentioned above, of all medical imaging procedures, the average
effective dose due to CT examinations is by far the highest. The radiation dose
varies between 15 mSv and 30 mSv. On average, this is about 150 times higher
than doses used in radiography examinations. Furthermore, these doses are very
dependent on the settings that are used during CT examinations. Radiation doses
increase with increasing field of view (FOV), tube voltage (kV), and the tube
current-exposure time product (mAs). Furthermore, multiple scans are often
required during patient follow-up, which causes the radiation burden to increase
rapidly.(16) The average effective dose related to CT examinations has been
increasing, and is now about six times higher than in the early 1970s.(12)
1.2.2.3 Cone beam computed tomography
Introduced at the turn of the 21st century, CBCT is a relative new member
of the family of medical diagnostic devices. CBCT is an innovative diagnostic
imaging technique in the field of dentomaxillofacial radiology (Figure 1.2).(22, 23)
Like CT devices, it allows for quick generation of detailed 3D images.(24, 25) CBCT
was specifically designed to produce cross-sectional images of the
dentomaxillofacial region. Due to its low cost and easy accessibility CBCT has
evolved rapidly. Today, it is used for implant planning, endodontics, orthodontics
and maxillofacial surgery.(26, 27) Exact numbers for the use of CBCT are currently
not available. However, a recent Belgian survey found that one out of five Belgian
Introduction
10
dentists has access to a CBCT device. It should be noted that only 9% of the
general dental practitioners and 12% of the orthodontists have direct access to a
CBCT device. On the other hand, over 60% of oral and maxillofacial surgeons and
periodontologists have direct access to a CBCT device.(28)
The radiation doses associated with CBCT examinations are intermediate to
those used by CT and radiography devices.(29-31) Typically, doses associated with
CBCT range from 0.01 mSv to 1.1 mSv per examination. As with CT devices, these
doses are highly dependent on the FOV, kV, and mAs.(32-36) Finally, the radiation
dose also increases rapidly with repeated exposure from multiple examinations.
Figure 1.2. Comparison of oral
radiograph (A.), oral cone-beam
computed tomography (B.) and oral
computed tomography (C.) images.
Radiography image:(37); Cone-beam
computed tomography image: Courtesy of
Prof. Dr. R. Jacobs (KU Leuven); Computed
tomography image: (38)
1.2.3 Radiation protection in medical imaging
As will be discussed in the next section (‘1.2.4 Health risks associated with
medical diagnostic procedures’), the use of IR in medical diagnostics is potentially
not without risk. These risks are expected to be even higher in children, since it
is known that children are more radiosensitive than adults. This is because the
tissues and organs in children are still growing and developing and fast-dividing
A. B.
C.
Introduction
11
cells are more sensitive to IR. Furthermore, the longer life expectancy of children
also plays a role. The longer you will live after IR exposure, the longer the time to
develop potential adverse health effects.(39) To control and limit these risks, the
principles of ‘justification’ and ‘dose optimization’ of radiation protection guidelines
(see also ‘1.4 Radiation protection: guidelines and risk assessment’) have been
defined for use in medical diagnostics.
Justification implies that a diagnostic procedure should only be performed
if its use results in more benefit than harm to the patient. Therefore it is important
to consider whether it is absolutely necessary to perform the imaging procedure.
The use of alternatives (e.g. magnetic resonance imaging) should always be
considered.(40)
Dose optimization is directly linked to the ‘as-low-as-reasonably-achievable’
(ALARA) principle. It focusses on minimizing radiation exposure to the patient.
Thus the IR dose that is used should be balanced between ALARA and the required
image quality for the intended use.(40) The IR dose, as well as the image quality,
mostly depend on the FOV, kV and mAs.(29, 36)
1.2.4 Health risks associated with medical diagnostic procedures
Exposure to IR is associated with (potential) health risks (see Chapter 1.3).
Although the use of CT, radiography and CBCT has undeniable benefits for the
patient, it is recognized that exposure to IR in medical diagnostics could have
drawbacks as well. As discussed in Chapter 1.4, exposure to IR increases the risk
of stochastic effects, or it can induce tissue reactions when the radiation dose is
above a certain threshold. The radiation doses used in medical diagnostics are not
high enough to cause tissue reactions, however there is a potential risk of inducing
stochastic effects. Currently, there is no conclusive data about the risks associated
with the low dose range. Therefore, understanding the health effects of radiation
doses associated with medical diagnostics is one of the major challenges in
radiation protection today.(41)
Radiation protection guidelines are important to protect the general public,
especially young children, from excessive IR exposure. It is well-known that
radiation sensitivity changes with age. Children are more radiosensitive than
adults.(39, 42) Therefore, questions were posed about radiation-induced health
effects, especially in children.(43-45) Most concern was raised about CT
examinations, since the doses there are the highest.(46) However, recently
concerns were expressed about the use of CBCT in children. The New York Times
published the article “Radiation Worries for Children in Dentists’ Chairs” in 2010
which clearly raised public awareness about radiation exposure to children.(47) In
this section available epidemiological data of patients on medical diagnostic
associated health risks will be discussed briefly.
Introduction
12
1.2.4.1 Epidemiological data on medical diagnostic exposure
The rapid increase in frequency of CT in medical diagnostics has led to
increased worries about the radiation dose. Therefore, several retrospective
epidemiological studies were conducted.(48) It has been estimated that the risk of
leukaemia increases following CT examinations in young children.(49-52) A positive
correlation between radiation dose and development of brain tumours later in life
was also described.(50-52) Huang et al. (2014) reported an association between
pediatric head CT and the risk of both benign and malignant tumour incidence.
They reported that the risk of developing a brain tumour increased 2.6-fold
following head CT.(53) Mathews et al. (2013) reported a 24% greater cancer
incidence in exposed children than in unexposed children.(51) Finally, it was
estimated that in the United States, all CT scans taken annually could cause up to
4870 future cancers.(49) Although valuable, these studies are criticized for several
reasons. Chief among them are the lack of individual dosimetrical data and a lack
of exposure information. Additionally, concerns were raised about reverse
causation.(54, 55)
The first epidemiological data concerning X-ray radiography dates back to
1958. It was reported that of the children who died of cancer before the age of
ten, the number of them that received radiographs was higher than in controls.(56)
After that, multiple studies have tried (and failed) to find a correlation between
cancer development and exposure to ionizing radiation due to radiography
examinations (reviewed in Mulvihill et al. (2017)).(48) Other studies, however, did
find a positive correlation between cancer development and exposure to
radiographs. In this context, exposure to radiographs has been linked to an
increased risk for Ewing’s disease, a rare sarcoma that usually occurs in/near
bones.(57) Furthermore, it has been reported that there is increased risk of
leukaemia and/or lymphoma.(58-63) One study also reported a correlation between
radiography and the incidence of brain tumours.(64) Finally, there are some studies
reporting an increased incidence of breast cancer following radiography.(65-67)
Despite a great number of studies failing to find a correlation between radiography
and cancer incidence, some of these studies indicate that even low doses such as
those associated with radiography can induce detrimental health effects. Finally,
these studies have also been criticized, mostly due to short follow-up periods and
the lack of proper exposure parameters and individual dosimetrical data.
To the best of our knowledge there is no epidemiological data linking CBCT
exposure to increased cancer incidence.
1.2.4.2 How to cope with limited data on health effects related to CBCT
examinations?
Today, there are no epidemiological data showing a connection between
CBCT examinations and increased cancer risk later in life. At best, some studies
Introduction
13
provided risk estimates based on radiation doses and simulations.(68, 69) These
studies estimate that in six cases per one million CBCT examinations, cancer will
develop later in life. Pauwels et al. (2014) estimated, based on skin dosimetry,
that in adults the incidence would be 2.7 cases per one million examinations,
whereas for children this would be 9.8 cases per one million examinations.(69)
Similar estimates were recently reported by Yeh et al. (2018).(68) Additionally,
there are only a few prospective studies aimed at investigating potential adverse
effects following CBCT examinations, whereas multiple studies were performed for
CT and radiological examinations (see Appendices 1 - 3). To the best of our
knowledge, only five have been conducted related to CBCT.(70-74) All of them found
increases in cytotoxicity markers after CBCT examination, but only two of them
found increases in genotoxicity markers (see Appendix 3). Thus, the available
data is inconclusive at this time. Furthermore, these studies did not specifically
study age-related differences.
To tackle these limitations, the overall aim of this thesis is to investigate
the biological effects of CBCT examinations in different age categories. Cellular
and subcellular changes following CBCT examinations in children and adults were
studied in dental stem cells, buccal mucosal cells, and saliva samples. As this is a
prospective study, that focusses mainly on acute changes, the emphasis is placed
on the DNA damage response, the DNA repair kinetics, and the (anti-)oxidative
stress response. This way, we hope to contribute to the current knowledge of
potential health risks associated with medical diagnostic imaging, specifically
CBCT examinations.
Introduction
14
1.3 Cellular and subcellular effects following ionizing
radiation exposure
Exposure to IR can have detrimental health effects. This has been clearly
shown by epidemiological data (see ‘1.4 Radiation protection: guidelines and risk
assessment’). Since the focus of this PhD thesis is on the cellular and subcellular
effects of X-rays used in medical diagnostic imaging, the next part will give an
overview of the interactions between X-rays and human cells/tissues.
1.3.1 Direct and indirect effects of ionizing radiation
X-rays transfer (part of) their energy to cells and biomolecules (e.g.
deoxyribonucleic acid (DNA), proteins and lipids) that make up the tissues that
they pass through. By transferring their energy directly to biomolecules, X-rays
can cause chemical changes, such as ionizations. This is called the direct effect of
IR.(75) Alternatively, IR can damage biomolecules indirectly by transferring its
energy to water molecules. This leads to the radiolysis of water, which generates
reactive oxygen species (ROS), thereby inducing oxidative stress (Figure 1.3).(76)
ROS are very reactive radicals and can cause sufficient damage to biomolecules
(e.g. DNA and proteins) to alter essential cellular functions.
Since more than 80% of a cell consists of water, most of the DNA damage
caused by X-rays is indirect.(16, 77) IR can cause several types of DNA lesions,
including single strand breaks (SSBs), double strand breaks (DSBs) and base
alterations. DNA DSBs are considered the most harmful because they are more
difficult to repair correctly.(78, 79) Inaccurate repair of DSBs could result in
mutations, chromosome rearrangements, chromosome aberrations and loss of
genetic information, which in turn can give rise to malignancies later in life.(80, 81)
Figure 1.3. Biological effects of ionizing radiation. Ionizing radiation can (a) directly
damage biomolecules by ionizing it or by breaking chemical bonds. Alternatively, it can (b)
radiolyse water, generating reactive oxygen species, which in turn will react with
biomolecules. This will result in indirect radiation-induced damage.(76)
Introduction
15
1.3.2 Oxidative stress
1.3.2.1 Generation and effect of reactive oxygen species by ionizing radiation
IR can generate free radicals or ROS through radiolysis of water.(77) ROS
generation takes place in three stages, all within nano- to microseconds: 1) the
physical stage, 2) the pre-chemical or physico-chemical stage and 3) the chemical
stage (Figure 1.4). During the physical stage, which occurs well within 10-12
seconds after the interaction with IR, the transferred energy from the IR leads to
excitation and ionization of water molecules (H2O* and H2O+, respectively), as well
as the formation of sub-excitation electrons (e-). These three newly formed
species then interact with each other and other nearby molecules. This happens
during the pre-chemical or physico-chemical stage. This stage occurs between 10-
15 and 10-12 seconds after IR exposure. An example of these reactions is the
formation of the hydroxyl radical (OH•): H2O+ + H2O H3O+ + OH•. Next, the
chemical stage takes place between 10-12 and 10-6 seconds after the initial IR
exposure. During this stage, the formed radicals will diffuse and react with
surrounding molecules, leading up to the biological stage.(77, 82) During the
biological stage, which takes place minutes or even years after the initial
exposure, important biomolecules are damaged by the newly formed ROS. ROS
can cause severe DNA damage by inducing DNA breaks, base damage, destruction
of sugars, cross links and telomere dysfunction.(83) OH• is the main actor, since it
is very effective in breaking chemical bonds. This damage can either be repaired,
in which case the cell survives, or the damage can be too extensive, which will
lead to cell death. However, if the damage is not correctly repaired, mutations can
occur. If these persist this could eventually lead to carcinogenesis.(77)
Introduction
16
Figure 1.4. Overview of the direct and indirect actions of ionizing radiation. Ionizing
radiation can directly damage important biomolecules such as the DNA. Alternatively, it can
damage them indirectly via the generation of reactive radicals through the radiolysis of water
molecules. The three phases of ROS formation are shown, namely the physical stage, the
pre-chemical (or physico-chemical) stage and the chemical stage. Finally the biological stage
occurs where the newly formed ROS interact with important biomolecules, leading to
damage to these molecules. Depending on the efficient repair of this damage, cells may die
or survive.(77)
OH• is the most prevalent radical, as well as the most potent at breaking
chemical bonds. It is highly reactive and causes harmful oxidations of cellular
components.(84) Other important ROS are hydrogen peroxide (H2O2) and the
superoxide anion (O2•-). The latter is, like OH•, very reactive. The former,
however, is less reactive. H2O2 is mildly oxidizing and mildly reducing, but it does
not readily oxidize biological molecules (i.e. DNA, lipids and proteins). The main
hazard of H2O2 is its ability to be converted into OH•, either by exposure to
ultraviolet light, or by interaction with one of several transition metal ions, iron
being the most important one. The latter will result in a Fenton reaction in which
OH• is formed.(85) ROS can cause oxidative damage to DNA, protein oxidation and
lipid peroxidation.(83, 84) Luckily, cells harbour an antioxidant defence system
against excessive ROS exposure. Only when the antioxidant defence system is
Introduction
17
saturated, ROS will be able to cause cellular damage. This imbalance between
oxidants and antioxidants in favour of the oxidants is called oxidative stress.(86)
1.3.2.2 Cellular defence mechanisms against oxidative stress
The antioxidant system is important for the redox homeostasis inside the
cell. It allows low levels of ROS to be present because at low concentrations, ROS
act as signalling molecules.(87) ROS can reversibly modulate several important
intracellular pathways that ensure the integrity and fitness of the cell.(86, 88) It has
been found that H2O2 for example is involved in microbial killing by macrophages
and neutrophils.(89). It is important to note that ROS that acts as signalling
molecule mostly comes from intracellular sources and is not induced by IR. IR
mostly induces OH• in higher, localized concentrations, whereas endogenous ROS
mostly comprises of O2•- and H2O2 in lower concentrations.(84) Endogenous sources
of ROS include the electron transport chain in mitochondria, nicotinamide adenine
dinucleotide phosphate oxidases, lipoxygenase, xanthine oxidase,
cyclooxygenase, cytochrome P450 monooxygenase, and nitric oxide synthase.
The delicate balance between signalling concentrations of ROS and harmful
concentrations of ROS, or redox balance, is vital for a normal cellular function.(88)
The redox balance is mostly maintained by an endogenous antioxidant system
that consists of 1) enzymatic antioxidants, 2) hydrophilic antioxidants, and 3)
lipophilic radical antioxidants. Hydro- and lipophilic antioxidants are also called
non-enzymatic antioxidants. They all have in common that they counteract free
radicals and neutralize oxidants.(90)
Enzymatic antioxidants include, amongst others, superoxide dismutases
(SOD), catalase (CAT) and glutathione peroxidases (GSH-Px) (Figure 1.5). They
are very effective against high levels of oxidative stress since they have the ability
to decompose ROS.(91) SOD is the major defence system against O2•- and exists
in three isoforms in humans: cytoplasmatic Copper/ZincSOD (SOD1),
mitochondrial SOD (SOD2) and extracellular Copper/ZincSOD (SOD3). SOD
dismutate O2•- to H2O2 and O2.(92) They are the first line of defence against ROS
and can be rapidly induced when oxidative stress is sensed.(86) CAT and GSH-Px
both neutralize H2O2 through reduction of H2O2 into water. By removing H2O2,
these enzymes prevent the formation of OH•, which is very reactive and damaging
to biomolecules. GSH-Px are a family of enzymes that are homologous to the
selenocysteine-containing mammalian GSH-Px1 enzyme. GSH-Px1 uses reduced
glutathione (GSH) as a co-substrate in the reduction of H2O2 to water. During this
reduction, GSH is oxidized to glutathione disulphide (GSSG). GSH is regenerated
through the reduction of GSSG by glutathione reductase, which therefore also is
important in the endogenous antioxidant system.(86, 93)
Introduction
18
Figure 1.5. Generation and metabolism of reactive oxygen species by enzymatic
antioxidants. Superoxide dismutases (SODs) convert superoxide anions (O2•-) to H2O2,
which in turn is reduced to water by catalase, glutathione peroxidases (GSH-Px) and
peroxiredoxins. In the presence of transition metals, H2O2 can spontaneously be converted
into the hydroxyl radical (OH•), which is extremely reactive. CAT, GSH-Px and peroxiredoxins
are therefore important in reducing the number of OH• molecules.(92) GSH = reduced
glutathione; GSSG = oxidized glutathione
Non-enzymatic antioxidants are low molecular weight, hydro- or lipophilic
molecules. Several vitamins are known to have antioxidant capabilities, which is
why they are frequently studied as food supplements for radiation protection
purposes. Vitamin A for example, which is produced in the liver, can bind to
peroxides and prevent peroxidation of lipids.(94) Vitamin C, on the other hand, is
effective in scavenging ROS, such as O2•-, H2O2, and, OH•.(95) Besides vitamins,
minerals are also dietary antioxidants. The most important minerals in this
regards are selenium and zinc. They are components of important antioxidant
enzymes (e.g. SOD and GSH-Px) and they are important for maintaining their
enzyme activity.(96, 97) Except for vitamins and minerals, there are many cellular
metabolites that exhibit an antioxidant function. Uric acid, for example is known
to prevent lipid and protein peroxidation.(98) Finally, flavonoids also exhibit
antioxidant activity. Their antioxidant activity depends on the arrangement of their
functional groups. Examples of flavonoids are phenolic acids and carotenoids,
which are mostly present in herbs, fruit, vegetables, seeds, and nuts.(86)
1.3.3 Oxidative stress measurements
Reactive radicals, such as ROS, have a very short half-life. This poses a
major problem when one wants to measure these ROS directly. Luckily, indirect
ways of assessing the impact of ROS exist.(99)
Currently, the only available technique that can be used to measure ROS
directly is electron spin resonance (ESR).(100) However, this technique is too
Introduction
19
insensitive to detect O2•- and OH• radicals in living systems. This can be overcome
by a process called ‘trapping’. In ‘trapping’, a radical will react with a trap molecule
(e.g. alpha-phenyl N-tertiary-butyl nitrone and 5,5-dimethyl-pyrroline N-oxide),
which results in one or more stable products. These products are then measured.
An example of ‘trapping’ is spin trapping. One major drawback is that the use of
traps perturbs the system under investigation. For example, if you want to
measure OH• and the damage it is causing, than trapping OH• molecules will
decrease the damage caused.(99) Therefore it might be better to opt for indirect
measurements of oxidative stress.
An indirect way of measuring ROS is through ‘fingerprinting’.
‘Fingerprinting’ techniques do not measure the ROS itself, but the damage that
they cause. For example, if ROS interact with a biomolecule and induce a
biochemical change to that molecule (i.e. a fingerprint), than the presence of that
fingerprint can be used to infer that ROS was generated. This approach uses
biomarkers to monitor the effects of antioxidants on oxidative stress, or the
induction of oxidative stress by certain agents.(101, 102) It should be noted that
currently there is no single biomarker for oxidative stress that meets all the criteria
of an ‘ideal’ biomarker of oxidative damage.(99) Examples of clinically used
biomarkers for the chemical impact of ROS that are relevant in the context of this
thesis, will be discussed next. Note that this overview is far from complete and is
discussed in more detail elsewhere (e.g. Halliwell and Gutteridge (2015) and
Frijhof et al. (2015)).(99, 103)
1.3.3.1 Oxidation of proteins
One approach to indirectly measure ROS, is to look at protein oxidation.
Protein carbonyls are formed when the protein backbones are oxidatively cleaved.
They can arise from several mechanisms, which explains their high concentration
in comparison to other ROS biomarkers.(104) Protein carbonyls can be detected
spectrophotometrically or by enzyme-linked immuno-sorbent assay (ELISA),
Western blot, immunohisto- or immunocytochemistry, or high-performance liquid
chromatography (HPLC).(103) They have been measured in blood and in
plasma.(103, 105) Additionally, reactions between arginine and lysine residues and
carbohydrates results in the formation of advanced glycation end products (AGEs).
AGEs can be measured through the use of antibodies. They can be detected
through ELISA, immunohisto- or immunocytochemistry, and Western blot.
Furthermore they can also be measured by HPLC. AGEs also have a specific
autofluorescence which can be used to detect them. They have been measured in
plasma or serum samples. However, due to the heterogeneity of AGEs, there is
no method that allows for measuring specific AGEs in a clinical setting.(103)
Introduction
20
1.3.3.2 Oxidation of lipids
Oxidized low-density lipoproteins (oxLDL) have been used for years as a
biomarker for cardiovascular disease (CVD).(106) OxLDL is mostly measured in
plasma samples. They can be detected by immunological techniques using
antibodies.(107) Linoleic and arachidonic acid are important targets for lipid
peroxidation by ROS. Examples of lipid peroxidation products are trans-4-
hydroxy-2-nonenal (4-HNE) and malonaldehyde (MDA). They can be detected by
several methods. Antibodies have used to detect them via immunocyto- and
immunohistochemistry but ELISA has been used as well.(108) 4-HNE has been
measured in plasma and serum samples.(108, 109)
1.3.3.3 Oxidation of DNA
Oxidative DNA damage occurs continuously in vivo. This oxidative damage
mostly takes place at the site of the purine guanine and is mostly caused by the
OH• radical.(99) Oxidized nucleotides are usually removed through nucleotide
excision repair (NER) or base excision repair (BER). Therefore the damaged
nucleotides are excreted into the extracellular space, after which they will leave
the body through excretion in urine.(103) Oxidative DNA damage is important since
it is widely accepted that it can contribute significantly to cancer development. It
could lead to misrepair of DNA, which could cause mutations.(110, 111) Thus
oxidative DNA damage could potentially be a biomarker that predicts cancer
development later in life.(99)
The most commonly measured oxidatively modified DNA base is 7,8-
dihydro-8-oxo-2’-deoxyguanosine (8-oxo-dG).(112, 113) It was first measured using
HPLC-based assays with sensitive electrochemical detection.(114) Later, ELISA
assays also became available for detecting 8-oxo-dG.(115, 116) Measuring 8-oxo-dG
has several important advantages: 1) the availability of sensitive detection
techniques, 2) it is formed by several ROS, including O2•- and OH•, and 3) it is a
mutagenic lesion. The latter indicates that it will be perceived by cells and that
mechanisms exist (i.e. NER/BER) to remove it. 8-oxo-dG has successfully been
measured in blood, urine and saliva.(117-123)
1.3.3.4 Markers of antioxidant defence
In theory, oxidative stress occurs when there is an imbalance between the
amount of oxidants and antioxidants. Therefore, it is likely that oxidative stress
can also be caused by, or aggravated by, an impaired antioxidant defence. As
antioxidants play an important role in ROS scavenging, it might be of interest to
Introduction
21
monitor antioxidant levels and/or activity. Concentrations of enzymatic
antioxidants (e.g. SOD1, CAT, and GSH-Px1) can be measured by ELISA or
Western blot, but their activity can also be monitored in saliva and blood
samples.(93, 124-130) Finally, antioxidants can also be monitored through gene
expression assays.(93, 130)
1.3.3.5 Oxidative stress after low dose radiation exposure
The link between low dose IR exposure and oxidative stress markers has
been studied before, mostly in blood samples.(131-133) Although, in recent years,
saliva has been recognized as a potentially useful bio-fluid in radiation protection
research, but has been scarcely investigated.(132, 134, 135) There are indications that
salivary levels of monocyte chemoattractant protein 1, interleukin-8, and
intracellular adhesion molecule 1 (all inflammation markers) are increased
following whole-body irradiation in cancer patients.(136) However, a lot of work still
needs to be done. Currently, no effects on salivary oxidative stress biomarkers
have been described following low dose IR exposure. Therefore, we will, for the
first time, monitor oxidative stress parameters (8-oxo-dG and antioxidant activity)
in saliva samples from adults and children following CBCT examination.
1.3.4 Radiation-induced DNA damage and the DNA damage response
As described in ‘1.3.1 Direct and indirect effects of ionizing radiation’, IR
can directly or indirectly cause a wide range of DNA lesions. Such lesions include
DNA breaks, both SSBs and DSBs, DNA cross links, base damage, base alterations
and destruction of the sugar phosphate backbone. Most of this DNA damage is
caused by ROS. Of the aforementioned lesions, DNA DSBs are considered the most
harmful, if not properly repaired.(79) If improperly repaired, DNA DSBs could result
in chromosome rearrangements, mutations, chromosome aberrations, loss of
genetic information, and, cell death. DSBs could cause genetic instability which
can be the onset for carcinogenesis.(80, 81) To cope with DNA damage, eukaryotes
have developed an efficient signalling network known as the DNA damage
response (DDR).(137) The DDR is a signalling cascade that responds to certain kinds
of DNA damage in order to repair it, or to induce apoptosis.
The DDR provides a mechanism for signal transduction from DNA damage
sensors to DNA damage mediators/transducers. These mediators/transducers
target a series of downstream effectors that will determine the outcome of the IR-
induced DNA damage. The main outcomes are 1) cell cycle arrest to provide time
for DNA repair, which in over 99% of the time occurs accurately, but could also
lead to misrepair, which could be the cause of mutations, or 2) cell cycle arrest
with no DNA repair leading to cellular senescence, or 3) induction of programmed
Introduction
22
cell death or apoptosis (Figure 1.6).(81, 138) Since DNA DSBs are considered the
most cytotoxic DNA lesion induced by IR, the DDR to DNA DSBs will be discussed
further in this section.
Figure 1.6. General overview of the DNA damage response. The presence of a DNA
double strand break is detected by a DNA damage sensor, which transmits the signal
downstream to a series of effector molecules through a signal transduction cascade of DNA
damage mediators/transducers. These will activate signalling mechanisms for either cell
cycle arrest and induction of DNA repair, or, when no repair occurs, cell death.(138)
1.3.4.1 DNA damage mediators
Radiation-induced DNA DSBs in eukaryotic cells are sensed quickly.
However, which proteins fulfil the function of ‘damage sensor’ is debatable. The
Mre11/Rad50/Nbs1 (MRN) complex, as well as Ku70/80 proteins, have been
described as having DNA damage sensing capabilities.(139, 140) These sensors
Introduction
23
activate several members of the phosphatidylinositol 3-kinase like kinase (PIKK)
serine/threonine protein kinase family. This protein kinase family includes ataxia
telangiectasia mutated (ATM), ataxia telangiectasia and Rad3-related protein
(ATR), and DNA-dependent protein kinase (DNA-PK), which become active
depending on the source of DNA damage and the timing.(141, 142) Both ATM and
DNA-PK are essential for the detection of DSBs, whereas ATR is necessary for
repair of single-stranded DNA regions that arise for example during replication
fork stalling.(143) Thus, for the repair of DSBs ATM and DNA-PK are the main
players. After activation, ATM has been shown to activate hundreds of proteins,
including p53-binding protein 1 (53BP1) and histone 2AX (H2AX) (Figures 1.6).
DNA-PK and ATR also has the ability to phosphorylate H2AX on serine 139
(γH2AX).(142, 144) γH2AX is one of the earliest DNA damage mediators that
becomes activated following DNA DSB formation. Accumulation of γH2AX at the
DSB site generates so called ionizing radiation-induced foci (IRIF) that provide a
binding site for downstream mediators in the DDR, such as 53BP1.(145) It is of
interest to know that visualization of both γH2AX and 53BP1 is increasingly being
used to monitor DSB formation and repair.(78, 140, 146-148) After IRIF formation,
γH2AX induces a positive feedback loop and serves as a binding site mainly for
the breast cancer 1 C-terminal (BRCT) domains of the mediator of DNA damage
checkpoint 1 (MDC1) protein.(149, 150) When MDC1 is positioned at the site of the
DSB, this creates a dock for other DNA repair proteins. That way, the MRN-ATM
complex is recruited to the DSB and ATM will phosphorylate other DNA damage
mediators, such as 53BP1 and breast cancer early onset 1 (BRCA1). At this point,
γH2AX serves as a signalling platform onto which all DDR proteins are
concentrated. This concentration of DDR proteins allows for amplification of the
original DNA damage signal.(151) The DNA damage mediators then transduce the
DNA damage signal to downstream effectors of the DDR (e.g. Checkpoint kinase
(Chk) 2 and p53 (Figure 1.6)(152)
1.3.4.2 DNA damage effectors
DNA damage sensors can activate DNA damage mediators, that in turn will
transduce the DNA damage signal to DNA damage effectors, including cell cycle
checkpoints (that allow for DNA damage repair), DNA repair pathways, and the
removal of severely damaged cells.
Cell cycle checkpoints
The main function of cell cycle checkpoints in eukaryotic cells is to detect
DNA damage, and allowing for this DNA damage to be repaired by slowing or
stopping the cell cycle (i.e. cell cycle arrest). The cell cycle depends on multiple
proteins, including cyclins, cyclin-dependent kinases (CDKs), and cyclin-
dependent kinase inhibitors (CKIs), which regulate the progression through the
Introduction
24
cell cycle.(153) Cyclins are the regulatory subunit of a heterodimer they form with
CDKs. CDKs, in turn, are the heterodimer’s catalytic subunit which will
phosphorylate several downstream targets upon activation by binding to its
respective cyclin. Through phosphorylation of target proteins, CDKs orchestrate
the cell cycle progression. Finally, CKIs inhibit the catalytic activity of CDKs,
resulting in cell cycle arrest. Several CKIs are therefore known as tumour
suppressor proteins, e.g. p16 and p21. Generally, CKIs cause cell cycle arrest in
the G1 phase to allow for DNA damage repair.(154, 155)
The cell cycle has three major checkpoints, namely the G1/S checkpoint,
the intra-S phase checkpoint and the G2/M checkpoint. At each of these
checkpoints, the cell has to assess if the genetic material is suited for cell division
or if DNA repair is needed. Activation of these checkpoints is regulated by CDKs
and CKIs.(156)
The G1/S checkpoint prevents cells from replicating when DNA DSBs are
detected in the G1 phase. This checkpoint is regulated by two pathways: 1)
through tumour suppressor p53, and 2) the checkpoint kinases (Chk) 1/Chk2-
Cdc25A-CDK2 pathway.(157) In short, activation of p53 in the nucleus will lead to
the induction of p21, which causes the cell to remain in the G1 phase by preventing
transition to the S phase.(157, 158) The Chk1/Chk2-Cdc25A-CDK2 pathway involves
the degradation of Cdc25A phosphatase, which results in rapid arrest of the cell
cycle at the G1/S checkpoint.(157, 158) Both pathways result in the inactivation of
CDK2, which inhibits the release of G1/S phase-promoting E2F transcription factor
from its bond to the retinoblastoma protein (RB).(159, 160) When E2F is bound to
RB, cell cycle progression is inhibited. Normally, when cell cycle progression is
needed, CDK2 will phosphorylate RB, which results in the release of E2F and
subsequent progression from the G1 phase to the S phase. Thus, the cell cycle is
halted in the G1 phase if CDK2 is inhibited and E2F is not released from RB.(161)
In the S phase, damaged DNA inhibits replication of DNA. This is known as
the intra-S phase checkpoint. It is regulated by two pathways: 1) the ATM/ATR-
Chk1/Chk2-Cdc25A pathway, and 2) the ATM-NBS1-SMC1 pathway.(162) As with
the G1/S checkpoint, ATM/ATR phosphorylate Chk1 or Chk2, resulting in the
phosphorylation and degradation of Cdc25A, which inhibits transition into the S
phase.(157, 162) On the other hand, ATM can phosphorylate NBS1, which eventually
leads to the activation of the intra-S phase checkpoint.(157)
Finally, there is the G2/M phase checkpoint. This checkpoint prevents the
cell from entering mitosis and thus transferring its (damaged) DNA to the next
generation of cells.(157) It is important to note that most cells are found to be most
sensitive to IR-induced DNA damage when they are in the G2 or M phase.(163)
Introduction
25
DNA damage repair pathways
IR can induce several types of DNA damage (as discussed above), all of
which rely on different DNA repair pathways. Simple lesions, such as SSBs and
base damage, can effectively be repaired through BER. Different ‘excision’ repair
mechanisms exist, such as NER and mismatch repair (MMR). However, these
pathways are less relevant in IR-induced DNA damage, since the most important
DNA lesion induced by IR are DNA DSBs.(159) In the following paragraphs, the
focus will be on two DNA repair pathways that are important in DSB repair: 1)
non-homologous end-joining (NHEJ), and 2) homologous recombination (HR). The
former is error-prone, but it is the most prominent pathway for DSB repair,
whereas the latter is error-free but in order to work it needs an intact homologous
template, which is not always present in severe DSBs.(159, 164)
NHEJ allows for DSB repair that can occur rapidly and throughout the entire
cell cycle, in contrast to HR. After detection of a DSB, the Ku70/80 complex binds
the ends of the DSB. This binding ultimately results in end processing of the DNA
strands, after which the DSB termini are ligated (Figure 1.7). This results in
complete repair of the DSB.(165) Although NHEJ efficiently repairs DSBs, it often
results in a loss of genetic information because at each end of the DSB a few
nucleotides are lost. However, NHEJ is the main DNA repair pathway in eukaryotes
and can be performed throughout the cell cycle, mostly in the G0 and G1
phases.(142, 166)
Introduction
26
Figure 1.7. Error-free homologous recombination (HR) compared to error-prone
non-homologous end joining (NHEJ). A. The DNA double strand breaks (DSBs) are
recognized and DNA damage mediators are activated. This initiates a cascade leading to
resection of the DNA strands. Next, a homologous strands is searched. When a homologous
sequence is found, DNA polymerase extends the single-stranded DNA. Then, the exchange
of damaged DNA strands occurs, resulting in the pairing of each damaged strand with its
homologous template. Finally, the damaged strands are extended and ligated, which results
in full repair of the DNA DSB. B. DNA DSBs are mainly repaired by error-prone NHEJ. When
a DSB is detected, the Ku70/80 complex binds the ends of the DSB. This binding ultimately
results in end processing of the DNA strands, after which the DSB termini are ligated.(167)
DNA DSB repair through HR is based on using an intact homologous DNA
strand as a template for DSB repair. By using a template strand, HR results in
error-free DSB repair, unlike NHEJ. However, the need for a template strand also
implies that HR can only occur when sister-chromatids are present, i.e. during the
late S phase and G2 phase. The DNA DSBs are recognized and DNA damage
mediators ATM and ATR are activated. This initiates a cascade leading to resection
of the DNA strands. Next, the repair mechanisms search for a homologous DNA
strand. When a homologous sequence is found, DNA polymerase extends the
single-stranded DNA. Then, the exchange of damaged DNA strands occurs,
resulting in the pairing of each damaged strand with its homologous template.
Introduction
27
Finally, the damaged strands are extended and ligated, which results in full repair
of the DNA DSB. (Figure 1.7).(156, 165, 168)
When DNA DSB are sensed, the cell has to decide via which of these
mechanisms the damage will be repaired. How they decide between NHEJ and HR
is not well understood so far. The cell cycle phase at the time of the DSB plays a
role, since HR can only occur in the late S- and G2 phases. Furthermore, 53BP1
and BRCA1 are thought to play a key role in deciding between NHEJ and HR.(169,
170)
Removal of severely damaged, non-functioning cells
If a cell is too damaged, or its genomic integrity cannot be guaranteed, cells
will be removed. As with DNA repair, several pathways for the removal of cells
exist. The four main modes of cell removal that can be induced by severe DNA
damage are senescence, apoptosis, necrosis, and autophagy (Figure 1.8).(171)
Figure 1.8. Overview of the four main modes of removing non-functioning cells
induced by DNA damage. Severely damaged DNA can evoke necrosis, autophagy,
apoptosis and senescence. The latter is not a form of cell death, but rather a state of stable
cell arrest. p53 plays a central role in the signal transduction following DNA damage. It is a
main actor in the apoptotic response and regulates the switch between senescence and
apoptosis. p21 is also an important mediator of senescence. Necrosis is mostly mediated
through ATP depletion and poly(ADP-ribose)polymerase (PARP) activation. Finally,
autophagy depends on the presence/absence of functional p53.(171)
Introduction
28
Cellular senescence is characterized by an irreversible growth arrest in the
G1 phase of the cell cycle. Due to this growth arrest, the proliferation of cells with
severe DNA damage is limited. This irreversible growth arrest can be caused by
several forms of cellular stress, such as activation of oncogenes, ROS and DNA
damage. Senescence that is caused by one of these stresses is called premature
cellular senescence.(172) Cellular senescence is considered an anti-proliferative
response and a tumour suppressor mechanism.(173) p53/p21 and p16/Rb are the
most important mediators of cellular senescence.(171) Activation of p53 following
DNA damage leads to the activation of CKIs such as p16 and p21. Inhibition of
CDK-cyclin complexes results in a cell cycle arrest, halting cellular proliferation.
Hypo-phosphorylated Rb is the most crucial component responsible for
senescence. Besides DNA damage, oxidative stress can also cause premature
cellular senescence. Oxidative stress, through an increase in ROS, activates the
p38 mitogen activated protein kinase (MAPK). Activated p38 MAPK leads to
increased transcriptional activity of p53 and upregulation of p21. p21 activation
results in inhibition of CDK-cyclin complexes leading to cell cycle arrest. A third
major component of damage-induced senescence is the senescence-associated
secretory phenotype (SASP). The SASP is mediated by nuclear facter κβ (NF-κβ)
and includes pro-inflammatory cytokines (e.g. interleukin-6 (IL-6) and IL-8),
chemokines, growth factors (e.g. transforming growth factor-β (TGFβ)) and
proteases.(174, 175) These proteins can cause inflammation in the vicinity of
senescent cells, but can also trigger senescence. TGFβ, for example, can trigger
senescence in a paracrine manner. The mechanism by which TGFβ achieves this
includes the generation of ROS and DNA damage, which ultimately leads to the
activation of p21 (Figure 1.9).(176)
Introduction
29
Figure 1.9. Overview of molecular pathways involved in damage-induced
senescence. DNA damage, reactive oxygen species (ROS) and the senescence-associated
secretory phenotype (SASP) all lead to activation of cell cycle inhibitors p21 and/or p16.
These, in turn, will inhibit important cyclin-dependent kinases (CKD), whose inhibition leads
to the inhibition of the retinoblastoma protein (Rb), which is the crucial component
responsible for senescence induction. Figure adapted from Muñoz-Espin and Serrano
(2014).(176) DDR = DNA damage response
Apoptosis is also known as ‘programmed cell death’. Like premature cellular
senescence, it is an response to cellular stress and occurs when DNA damage
repair is slow or incomplete.(171) It is therefore an important mechanism for
maintaining homeostasis and for removing cells during different developmental
processes. It also limits the number of cells that have damaged DNA, which could
lead to an accumulation of mutations that could lead to carcinogenesis. This way,
apoptosis is a cellular mechanism that help to prevent tumour formation.(177, 178)
When a cell goes into apoptosis, morphological changes occur. These changes
include peripheral condensation of nuclear DNA without disassembly of the nuclear
envelope, plasma membrane blebbing, and cleavage of the nucleus into
membrane-enclosed structures, which are known as apoptotic bodies.(179) Based
on these morphological changes, apoptosis can be distinguished from necrosis
(see below). Unlike necrosis, apoptosis does not result in the release of
intracellular components into the extracellular space.(171) Since apoptosis is
‘programmed’, it is no surprise that it is a complex process that can proceed
through at least two major pathways: the extrinsic and intrinsic pathways (Figure
Introduction
30
1.10).(180) Both pathways can be induced by severe or unrepairable DNA
damage.(177) Activation of members of a family of cysteine aspartyl proteases
(caspases) is a hallmark of apoptosis. They play a central role during the
execution-phase of apoptosis, and they can amplify the apoptosis signal though
caspase cascades. Caspases 8 and 9 are mostly regulators of apoptosis, whereas
caspases 3, 6, and 7 are important effectors of apoptosis.(181) The extrinsic
pathway relies on death ligands that bind to the death receptors on the cell
surface. This leads the activation of caspase 3, which is an important effector
which starts a caspase cascade that eventually leads to apoptosis. The intrinsic
pathway, which is typically initiated by severe DNA damage, starts with the
activation of p53. If Bax is activated by p53, it causes the release of cytochrome
c from the mitochondria, which leads to the activation of caspase 3 by caspase 9,
after which caspase 3 starts the caspase cascade that leads to apoptosis, similarly
to the extrinsic pathway.(182)
Necrosis is an acute form of cell death, usually following rapid energy, i.e.
ATP, loss. Necrosis is mostly an unregulated form of cell death, however evidence
shows that it could be regulated by poly(ADP-ribosyl)ation. Other proteins
potentially involved are among other p53, p21 and DNA-PKcs.(183) Besides rapid
energy loss, necrosis also occurs following direct cellular trauma. Eventually
necrosis results in loss of cell membrane integrity and release of intracellular
components into the extracellular space, which could induce a local inflammatory
reaction.(184)
Autophagy, which translates to ‘self-eating’, is a well-known catabolic
mechanism for the degradation of proteins and other subcellular components by
lysosomal proteolysis. It could be triggered in response to several stress stimuli,
including DNA damage. Autophagic cell death is characterized by the presence of
autophagic structures and, unlike in apoptosis, chromatin condensation in the
later stages of the autophagic process. The process of autophagy may be
regulated by p53.(171, 185)
Introduction
31
Figure 1.10. Extrinsic and intrinsic apoptotic pathways. The extrinsic pathway (left)
relies on death ligands that bind to the death receptors on the cell surface. This leads to the
conversion of inactive pro-caspase 8 into the active caspase 8. Caspase 8 then activates
caspase 3, which starts a caspase cascade that eventually leads to apoptosis. The intrinsic
pathway (right) is initiated by the activation of p53. p53 activates its apoptosis-related
target genes, such as Bcl-2 associated X (Bax), Bcl-2 homologous antagonist killer (Bak),
p53 up-regulated modulator of apoptosis (Puma), and apoptotic protease activating factor
1 (Apaf1). If Bax is activated by p53, it causes the release of cytochrome c from the
mitochondria. Cytochrome c and Apaf1 then form the apoptosome. The apoptosome than
activates pro-caspase 9 to form caspase 9. Finally, caspase 9 activates caspase 3 and
caspase 3 then start the caspase cascade that leads to apoptosis. Note that the extrinsic
and intrinsic pathway intersect at the level of caspase 3.(182)
1.3.5 DNA damage measurements
There are a lot of different assays available that can be used to assess IR-
induced DNA damage, DNA damage repair, and the cellular outcome. One can
look at DNA damage induction, cell survival, chromosomal aberrations, etc.. In
vivo, mostly cytogenetic assays are performed, since they are well-established.
Examples are the dicentric chromosome assay, chromosome aberrations, and
micronucleus (MN) assay. Besides these cytogenetic assays, there are assays that
Introduction
32
focus on DNA damage and repair (kinetics), such as the comet assay, pulsed-field
gel electrophoresis and the γH2AX assay. Finally there are assays that focus on
cellular endpoints, including cell cycle assays and senescence assays. Although
the dicentric chromosome assay, MN assay, and comet assay have been used
before to study cellular effects following medical diagnostic imaging (see
Appendices 1, 2 and 3), the focus of this section will be on the γH2AX/53BP1
assay, cell cycle analysis and senescence assay, as these assays were performed
during this PhD research.
1.3.5.1 Assessing DNA damage and repair through the γH2AX/53BP1 assay
As described in ‘1.3.4 Radiation-induced DNA damage and the DNA damage
response’, phosphorylation of H2AX at the site of DNA DSBs leads to the formation
of γH2AX foci. The maximum number of foci is detectable 30 minutes to one hour
after IR exposure. Depending on the cell type and the radiation dose, the number
of foci usually decreases to baseline levels within a few days, mostly within 24
hours.(186, 187) Thus, counting γH2AX foci at different time points is an endpoint
that can be used to assess the formation and repair kinetics of DNA DSBs following
IR exposure. Similarly, 53BP1, after phosphorylation and activation, forms foci at
the site of DNA DSBs.(79) Both γH2AX and 53BP1 foci show a quantitative
relationship between the number of foci and the number of DSBs that are
present.(140, 188, 189) Therefore, one γH2AX and/or 53BP1 focus represents one or
several clustered DNA DSBs.(78, 190)
γH2AX and/or 53BP1 foci are most frequently scored via immunocyto-
and/or immunohistochemistry followed by fluorescence microscopic analysis.
Alternatively, flow cytometry can be used to detect fluorescence intensity.(191)
Microscopically, foci can be counted by manual scoring through the eye piece or
of digital images, or by automated scoring by using image scoring software.(187)
Automated scoring has several advantages over manual scoring such as the
potential for high throughput, exclusion of scorer subjectivity and elimination of
the time-consuming counting process. Flow cytometrical analysis, on the other
hand, has the advantage that is faster than microscopic analysis. However, it is
less sensitive because it cannot discriminate foci from background staining
spots.(144) This is important following low dose exposure, such as those used in
diagnostic radiology, where sensitive scoring is required. γH2AX increases has
been detected following radiography and CT examination, thus it can be detected
following IR exposure of a few mSv and even less (see Appendices 1 & 2).
γH2AX is formed following phosphorylation of H2AX during the DDR.
However, even in the absence of DSBs artefactual γH2AX foci can be formed. This
could be due to non-specific immunostaining or formation of antibody aggregates
during the staining process. It has been hypothesized that anti-γH2AX antibodies
Introduction
33
could bind to parts of Golgi vesicles and/or the endoplasmatic reticulum.(186)
γH2AX can also be observed in the S phase of the cell cycle.(78, 137) Furthermore,
γH2AX foci can also be detected in early apoptotic DNA breakage.(137)
To reduce the impact of these artefactual γH2AX foci, a double
immunostaining for both γH2AX and 53BP1 can be used. This enhances the
sensitivity of the microscopic analysis of DNA DSBs. It was shown that γH2AX IRIF
co-localize very reliably with 53BP1 IRIF.(79, 146, 192-194) An added benefit is that
53BP1 does not co-localize with γH2AX in early apoptotic cells. Therefore the
quantification of co-localized γH2AX/53BP1 foci can rule out misclassification of
early apoptotic DNA breakage, which would induce γH2AX foci, but not 53BP1
foci.(195, 196)
1.3.5.2 Cell cycle analysis
One of the main cellular outcomes following (severe) DNA damage, is the
induction of cell cycle arrest (Figure 1.11). This arrest allows the cell time to repair
the damage. Monitoring of the cell cycle can be performed through gene
expression analysis of genes that are essential for the cell cycle. Similarly,
proteomic techniques can be used to monitor levels of important cell cycle
mediators (e.g. p53 and p21). Finally, flow cytometry is frequently used to analyse
the specific cell cycle phases.(148) Flow cytometrical analysis of cell cycle
progression is relatively simple and uses an intercalating DNA dye alongside a
nucleotide analogue, which can be detected via antibodies (e.g. 5-bromo-2'-
deoxyuridine (BrdU)). Whilst an intercalating DNA dye allows distinction between
the G1/G0, S, and G2/M phases based on nucleic acid content, addition of the stain
of a nucleotide analogue allows for a clearer distinction between the G1/G0 and
G2/M phases on the one hand, and the S phase on the other hand. Since the
nucleotide analogue is incorporated into newly synthetized DNA, only S phase cells
will stain positive when using anti-nucleotide analogue antibodies, resulting in a
clearer distinction than solely relying on a DNA dye. (148, 197)
Introduction
34
Figure 1.11. Overview of the cell cycle. The cell cycle is composed of four main phases:
1) the G1 phase, or cell growth phase, 2) the S phase, or DNA replication phase, 3) the G2
phase, or mitosis preparation phase, and 4) the mitosis phase. The mitosis phase itself
consists of the prophase, metaphase, anaphase, and telophase. These phases make up the
actual cell division.(198)
1.3.5.3 Premature cellular senescence
If a cell suffers severe DNA damage, it could become senescent. This is
characterized by an irreversible cell cycle arrest.(199) Therefore, markers of cell
cycle arrest can be used to assess if a cell became senescent prematurely. The
hallmark of senescent cells is the increase in β-galactosidase activity. This has led
to the development of the X-gal assay, which is based on the increased β-
galactosidase activity. It is a microscopic assay, which allows for detection of
senescent cells.(173, 200) Senescent cell can also be detected through analysis of
the SASP. Cytokines such as IL-6, IL-8, insulin-like growth factor binding proteins
2 (IGFBP-2), and IGFBP-3 can be detected using proteomic approaches, as well
as genomic techniques.(174, 201) IL-6 and IL-8 are pro-inflammatory cytokines,
which are associated with DNA damage and which can cause (persistent) cell cycle
arrest through paracrine and autocrine signalling.(202) IGFBP-2 and IGFBP-3 are
proteins to which insulin-like growth factors (IGF) are bound. When bound to
IGFBP-2 and IGFBP-3, they cannot interact with their receptor, leading to
inhibition of cell growth which is generally induced by IGF.(203, 204) Because of their
ability to sequester IGF and thereby inhibiting cell growth, IGFBP-2 and IGFBP-3
have been studied as markers for cellular senescence. Indeed, both increased
levels of IGFBP-2 and -3 have been found to be associated with senescence.(202,
205, 206)
Introduction
35
1.4 Radiation protection: guidelines and risk
assessment
Exposure to IR can cause detrimental effects, certainly after exposure to
high doses. These detrimental effects are either tissue reactions (formerly known
as deterministic effects) or stochastic effects. Almost immediately after the
discovery of X-rays in 1895 tissue reactions were observed.(207-209) They are
associated with doses above 100 milligray (mGy) and occur within hours up to a
few weeks, sometimes even up to several years. Examples are skin burns
following radiotherapy and cataract. For tissue reactions, a threshold dose exists
below which no tissue reactions are observed.(210) Stochastic effects (e.g.
radiation-induced carcinogenesis) are mostly associated with low doses of IR,
which are defined to be lower than 100 mGy, but are also observed following high
IR doses. They occur over a longer period of time then tissue reactions (i.e.
months up to several years). The ICRP aims to protect people from radiation-
induced stochastic effects by advising on radiation protection guidelines and
regulations.(10, 211)
One of the greatest challenges in radiation protection today is determining
the detrimental effects of exposure to doses lower than 100 mGy, i.e. the
stochastic effects. For doses higher than 100 mGy, epidemiological studies
support a linear-no-threshold (LNT) model. These epidemiological studies that
validate this LNT model include studies with atomic bomb survivors, medically and
occupationally exposed populations and environmentally exposed groups (e.g.
people living in Ukraine following the Chernobyl disaster).(212) Policy makers use
models based on these data to estimate the stochastic effects (i.e. risks)
associated with exposure to doses lower than 100 mGy. These models include the
LNT model, the threshold model, the hormetic (or adaptive) model and the
hypersensitivity model (Figure 1.12).
The LNT model is currently used by policy makers for cancer risk estimation
following exposure to low doses of IR. It assumes that for every dose a person is
exposed to, there is a proportional increase in detrimental effects, such as cancer
risk. The LNT model also assumes that there is no threshold dose below which no
detrimental effects occur. However, other models exist in this low dose range such
as the threshold model that assumes that a certain threshold dose must be
exceeded in order to initiate a biological response. Per definition, no detrimental
effects are expected to occur below this threshold dose. Note that this model
resembles the model for tissue reactions, in which also a threshold dose must be
exceeded before tissue reactions occur. Thirdly, there is the hormetic model. This
model suggests that exposure to low doses of IR could induce beneficial effects,
leading to a radio-adaptive response, resulting in reduced risk.(213) Finally, the
Introduction
36
hypersensitivity model suggests that our cells/tissues are hypersensitive to very
low doses of IR, thus leading to greater biological risks in the low dose range.(214)
Thus far, there is a lack of evidence to definitely prove or disprove these models.
Epidemiological data supports the LNT model, but only above 100 mGy. For doses
lower than 100 mGy there is no clear consensus due to a lack of statistical power
of the epidemiological data.(215-220) This has led to criticism on the LNT model in
recent years, since there is increasingly more evidence that disproves this model
in the low dose range.(218, 219, 221)
Figure 1.12. Graphical representation of the different models explaining the dose-response relationship in the low dose range. Four models are represented that show potential dose-response relationships for radiation exposure below 100 milliGray. The linear-no-threshold model (black line), the linear-threshold model (pink line), the hermetic model (green line) and the hypersensitivity model (red line). As depicted by the linear part of the curve, the effects associated with doses higher than 100 milliGray are well understood thanks to epidemiological data that is available from the Hiroshima and Nagasaki bombings, as well as the Chernobyl disaster.
Introduction
37
1.5 The oral cavity
The oral cavity is a region in the human body that is comprised of the lips,
hard palate, soft palate, the retromolar trigone, the front two-thirds of the tongue,
the gingiva, teeth, buccal mucosa (BM), and the floor of the mouth under the
tongue (Figure 1.13).(222) This relatively small region contains a lot of different
tissues, for example muscle tissue, bone and cartilage tissue, and glandular
tissue. (222) To study the effects of low dose IR exposure due to CBCT examinations
in the dentomaxillofacial region, however, this thesis is limited to the study of
dental stem cells, buccal mucosal cells (BMCs), and saliva samples.
Figure 1.13. Overview of the anatomy of the oral cavity.
1.5.1 Dental stem cells
Recently, teeth have been described as the most natural, non-invasive
source of mesenchymal stem cells (MSCs). Indeed, teeth harbor several types of
MSCs. A major breakthrough was achieved in 2000, when Gronthos et al.
identified and isolated progenitor cells form the dental pulp from adults. These
cells were aptly dubbed dental pulp stem cells (DPSCs).(223) Later on, dental pulp
stem cells were also extracted from deciduous teeth (SHEDs).(224) Since then,
stem cells were also isolated from the apical papilla (SCAPs), the dental follicle
(DFSCs), and the periodontal ligament (PDLSCs) (Figure 1.14).(225-227)
Introduction
38
DPSCs and SHEDs, which are both MSCs originating from the dental pulp,
have the capability to differentiate into odontoblasts. This is important in vivo,
since mature odontoblasts cannot repair damaged dentin. Thus when dentin is
damaged, DPSCs migrate from the dental pulp to the dentin surface, where they
differentiate into odontoblasts. These newly formed odontoblasts will then produce
reparative dentin, which is of poorer quality than the primary dentin, but still
provide protection to the dental pulp.(228, 229) Therefore, DPSCs are thoroughly
investigated as a natural way of repairing teeth by using DPSCs to produce dental
tissues.(230, 231)
SCAPs are related to developing tooth roots. It has been shown that the
presence of SCAPs is required for the continuation of root maturation.(226)
Furthermore, it was reported that SCAPs are superior to DPSCs when it comes to
plasticity, and versatility.(232) As a tool in tissue engineering, SCAPs are being
studied for their odontogenic, osteogenic, angiogenic, and neurogenic capabilities.
Finally, because of their angiogenic and neurogenic capabilities, they are also of
interest for wound healing and treatment of neurodegenerative diseases.(233)
Finally, DFSCs are found in the connective tissue that surrounds the
developing teeth. Like DPSCs/SHEDs and SCAPs, they are associated with tooth
development. They have the ability to differentiate into osteoblasts, chondrocytes,
and adipocytes in vitro. Furthermore, they can form calcified nodules, indicating
that they can differentiate into cementum. Finally, they can also generate
periodontal ligament. Their differentiation capabilities are similar to those of
SCAPs.(234) DFSCs are currently investigated for tooth root regeneration.(235)
Figure 1.14. Overview of the different types of dental stem cells and their in vivo
location. Figure adapted from Sharpe (2016).(236)
Introduction
39
1.5.2 Buccal mucosal cells
The BM is a mucous membrane that lines the inside of the cheeks. It is a
specialized, non-keratinized stratified squamous epithelium. This entails that the
BM consists of several layers of cells that rest on connective tissue. The outer
layers lose their nuclei, and slough off due to sheer stress in the mouth. The basal
cells, on the other hand, serve as progenitor cells for the upper layers.(237)
BMCs have several major functions in the oral cavity. The most prevalent
one is that of primary protection of the BM against external aggressors such as
microorganisms and toxic substances. Furthermore, they can also secrete several
classes of inflammatory mediators.(238) Because of this barrier function, BMCs are
useful for studying the effects of exposure to environmental agents.(239, 240)
BMCs are easy to use as a biological sample, since they are easily accessible
and they can be collected in a minimally invasive way.(237) They are being
increasingly used for research and diagnosis. They have been used, for example,
to diagnose diseases such as Prader-Willi syndrome, for verifying the increasing
risk for gonadoblastoma in Turner syndrome patients, and to estimate cancer risk
through detection of the HNF1B gene mutation.(241-243) Finally, they are also used
to study the effects of exposure to genotoxins, such as IR.(239, 244-246)
1.5.3 Saliva
Saliva is the whole fluid present in the oral cavity. It originates mainly from
three major salivary glands: the parotid, submandibular, and sublingual glands.
Small portions originate from minor salivary glands, gingival crevicular fluid
containing bacteria, BMCs, erythrocytes, leukocytes, and food debris.(247)
In vivo, its main functions are providing protection to and maintain the
integrity of the BM. It does this through lubrication, buffering action, and
antibacterial and antiviral activity. Finally, saliva is also important in food digestion
as it contains digestive enzymes.(247) Over 1000 salivary proteins have been
described so far. Most of these can also be found in plasma. Besides proteins,
saliva also contains electrolytes, immunoglobulins, metabolites, hormones, and
vitamins.(248-250) Because of this, saliva is often referred to as ‘the mirror of the
body’.(251)
Like BMCs, saliva samples are easy to collect. It can be collected non-
invasively and painlessly.(249) Since the early 1990s, it is increasingly studied as a
diagnostic fluid.(251, 252) Since then, it has been found that it is a suitable biofluid
for –omics and disease studies.(122, 247) Genomic DNA has been isolated from saliva
and has been used in several genomic studies, such as genome-wide
microarrays.(253) It has also been used for clinical genetic testing,
pharmacogenomics testing, diagnostic DNA testing, and population studies.(247)
Furthermore, salivary proteomics has been used to detect several diseases, such
Introduction
40
as cardiovascular disease, type II diabetes mellitus, and squamous cell
carcinoma.(254-257) Additionally, salivary metabolomics has gained attention as a
disease diagnostic, stratification, and early detection tool. The salivary
metabolome provides a ‘snapshot’ of gene function, enzyme kinetic activity, and
changes in metabolic reactions. For example, mass spectrometry studies have
identified metabolites as biomarkers for oral squamous cell carcinoma.(258) Most
of these –omics studies were reviewed by Nunes et al. (2015).(247) Finally, saliva
has been used to study biomarkers of both high and low IR dose exposure.(134-136,
259)
Introduction
41
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Chapter 2: Scope and aim of the research
55
Nowadays, one of the prime challenges in radiation protection is assessing
the possible biological effects of exposure to low doses of IR. Unfortunately, data
are only conclusive for exposure to doses above 100 mGy. Although the ICRP aims
to protect people from radiation-induced stochastic effects by advising on
radiation protection guidelines and regulations, conclusive data on low dose (i.e.
below 100 mGy) health effects remain elusive.(1-8) Data on low dose effects,
however, are of importance in medical imaging applications of IR, such as CT and,
more recently, CBCT, which typically use doses far below 100 mGy (9-12).
CBCT is a relatively new and innovative diagnostic imaging technique
introduced in oral health care at the turn of the century.(13, 14) Its growing use lies
in the diagnostic potential related to the transition from two-dimensional to three-
dimensional dentomaxillofacial diagnostic imaging.(15-18) CBCT technology has
rapidly evolved in the past decade. Nowadays it has become a widely available
diagnostic tool for clinicians and has therefore found applications in multiple dental
specialties, including implant planning, endodontics, orthodontics and
maxillofacial surgery.(13, 14, 16, 19-22) CBCT relies on X-rays for its image acquisition.
As in CT, the absorbed IR dose in CBCT heavily depends on selectable exposure
parameters that determine the image quality such as kVp, mAs, field of view
(FOV), amount of 2D projections, reconstitution algorithm, etc..(10-12, 16, 23)
Therefore, a wide range of CBCT doses is observed, typically ranging from about
0.010 to 1.100 mSv per examination.(10, 11, 23-27) CBCT doses are lower than CT
doses (organ dose of about 15 mSv), however, they are higher than classical 2D
dental radiography techniques (organ dose of 0.001 – 0.1 mSv).(12, 16, 28-31)
IR is capable of damaging biomolecules (e.g. DNA or proteins) directly or
indirectly via the hydrolysis of water which generates free radicals, such as
ROS.(32, 33) Since more than 60% of a cell consists of water, most of the damage
is caused indirectly via ROS (e.g. the hydroxyl radical, superoxide radicals and
hydrogen peroxide).(30, 34) An excess of ROS causes oxidative stress. In the
context of oral pathology, oxidative stress is associated with periodontitis, dental
caries and oral cancers.(35, 36) ROS can cause oxidative DNA damage through
oxidative base lesions, of which over 20 different ones have been identified.(37) An
example is 8-oxo-7,8-dihydro-2’-deoxyguanosine (8-oxo-dG), a mutagenic base
modification.(38) Other types of DNA lesions include single strand breaks, double
strand breaks (DSBs) and base alterations.(34, 39) DSBs are the most critical DNA
lesions caused by IR. When not repaired correctly, DSBs can lead to chromosome
rearrangements, mutations and loss of genetic information.(40-45) To protect
themselves, eukaryotic cells have developed the DNA damage response (DDR), a
set of signalling and DNA repair pathways.(46-48) The DDR consists of a signalling
cascade that results in the recruitment of multiple DDR proteins to the vicinity of
DSBs, including histone H2AX phosphorylated on serine 139 (γH2AX) and p53-
binding protein 1 (53BP1). Both γH2AX and 53BP1 form DNA damage foci and
show a quantitative relationship between the number of foci and the number of
DSBs (49, 50).
Chapter 2: Scope and aim of the research
56
Because of the potential adverse health effects of IR exposure clinical
studies following patients after medical diagnostic imaging procedures have been
performed. Multiple controversial studies indicate that exposure of children to
diagnostic radiology may lead to radiation-induced malignancies later in life.
Retrospective studies observed that the use of CT scans in children could triple
the risk of leukaemia and brain cancers (51-53). Furthermore, it was estimated that
the probability to develop radiation-induced malignancies after CBCT exposure is
6 cases per 1,000,000 CBCT scans on average (54-56). Despite these potential links
between diagnostic radiology and radiation-induced malignancies, absolute
evidence from prospective studies is scarce (4, 9). Given that children are more
radiosensitive than adults, this raised questions about potential radiation-induced
health effects associated with diagnostic radiology in children (10, 11, 57-60). IR doses
associated with paediatric dental CBCT became a major concern for the general
public when the New York Times published two articles about the topic (2010 and
2012).(61, 62) Most CBCT examinations are performed on children (< 18 years old),
mostly during orthodontics, but also during pedodontic procedures.(10, 60)
Currently, epidemiological studies are lacking for CBCT exposure. As a
consequence, researchers have to rely on radiobiological evidence as well as
prospective studies that monitor current patients, rather than historical cohorts.
Radiobiological research can help to gain more insights into the underlying
mechanisms.(1, 63)
The overall aim of this thesis was to investigate the biological effects of low
dose IR associated with dental CBCT in different age categories. Emphasis was
placed on 1) the DNA damage response and repair kinetics following low dose IR
exposure through immunofluorescent staining for γH2AX and 53BP1, and 2) the
(anti)oxidative stress response following low dose radiation exposure. These
parameters were monitored in dental stem cells, buccal mucosa cells (BMCs),
and/or saliva samples collected from pediatric and adult patients prior and after
CBCT examination.
Dental stem cells are mesenchymal stem cells that reside inside, or closely
to, the teeth. Several types of stem cells have been identified since the early
2000s: dental pulp stem cells (DPSCs), dental pulp stem cells from deciduous
teeth (SHEDs), stem cells from the apical papilla (SCAPs), dental follicle stem cells
(DFSCs), and periodontal ligament stem cells (PDLSCs).(64-68) All of them have
crucial functions in tooth development and repair.
The buccal mucosa (BM), which lines the oral cavity, is an easily accessible
source for collecting BMCs in a minimally invasive, pain-free way.(69) BMCs have
been used to study (amongst others) the impact of nutrition, lifestyle factors and
exposure to genotoxins, including exposure to IR.(70, 71) IR-induced genotoxicity
can be monitored in BMCs by measuring γH2AX levels and can be used to monitor
radiation exposure and DNA damage in radiotherapy patients.(72, 73)
Chapter 2: Scope and aim of the research
57
Saliva is a bodily fluid that is secreted into the oral cavity. It originates
mainly from the parotid, submandibular and sublingual glands and is an aqueous
solution (> 99% water) containing both organic and inorganic molecules.(74)
Saliva, commonly referred to as ‘mirror of the body’, has several advantages over
other biological samples, such as blood. It is readily available, collection can be
done in a non-invasive way, and its use is very cost-effective.(75, 76) These
advantages make saliva an ideal sample to collect from paediatric patients and
for use in diagnostics.(76, 77) Currently, salivary diagnostics is becoming
increasingly important in radiation biomarker research.(75, 78) Since X-rays induce
most damage to biomolecules via ROS, measuring ROS and their effects in saliva
samples are good indicators of radiation exposure.
Firstly, the protocols for the detection of DNA DSBs in BMCs, and 8-oxo-dG
and total antioxidant capacity in saliva samples were optimized and validated
before use in pediatric patients (Chapter 3). Next, DNA damage induction and
repair were studied ex vivo in buccal mucosa cells obtained from adults and
children following dental CBCT. Simultaneously, we monitored oxidative damage
by measuring 8-oxo-dG levels in saliva samples from the same cohort of patients
(Chapter 4). In Chapter 5, in vitro experiments with paediatric dental stem cells
were performed in which the γH2AX/53BP1 assay for DNA damage induction and
repair, cell cycle progression, and premature cellular senescence were analysed.
Finally, time-dependent antioxidant responses were monitored in buccal mucosal
cells and saliva samples from patients following CBCT examination (Chapter 6).
Our experimental data provide insight into the cellular and subcellular changes
that occur after low dose IR exposure, both in patients of different age categories
exposed to dental CBCT, as well as in vitro. These data may eventually contribute
to the improvement of radiation protection guidelines and regulations by
introducing age-specific guidelines for medical diagnostic radiology.
Chapter 2: Scope and aim of the research
58
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limitations and optimization. Cell cycle (Georgetown, Tex). 2010;9(4):662-9. 40. Dugle DL, Gillespie CJ, Chapman JD. DNA strand breaks, repair, and survival in x-irradiated mammalian cells. Proc Natl Acad Sci U S A. 1976;73(3):809-12. 41. Olive PL. The role of DNA single- and double-strand breaks in cell killing by ionizing radiation. Radiat Res. 1998;150(5 Suppl):S42-51. 42. Jackson SP. Sensing and repairing DNA double-strand breaks. Carcinogenesis. 2002;23(5):687-96. 43. Richardson C, Jasin M. Frequent chromosomal translocations induced by DNA double-strand breaks. Nature. 2000;405(6787):697-700. 44. Vamvakas S, Vock EH, Lutz WK. On the role of DNA double-strand breaks in toxicity and carcinogenesis. Crit Rev Toxicol. 1997;27(2):155-74.
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45. Khanna KK, Jackson SP. DNA double-strand breaks: signaling, repair and the cancer
connection. Nat Genet. 2001;27(3):247-54. 46. Kinner A, Wu W, Staudt C, Iliakis G. Gamma-H2AX in recognition and signaling of DNA double-strand breaks in the context of chromatin. Nucleic Acids Res. 2008;36(17):5678-94. 47. Riches LC, Lynch AM, Gooderham NJ. Early events in the mammalian response to DNA double-strand breaks. Mutagenesis. 2008;23(5):331-9. 48. Ciccia A, Elledge SJ. The DNA damage response: making it safe to play with knives. Mol Cell. 2010;40(2):179-204. 49. Goodarzi AA, Jeggo PA. Irradiation induced foci (IRIF) as a biomarker for radiosensitivity. Mutat Res. 2012;736(1-2):39-47. 50. Asaithamby A, Chen DJ. Cellular responses to DNA double-strand breaks after low-dose gamma-irradiation. Nucleic Acids Res. 2009;37(12):3912-23. 51. Pearce MS, Salotti JA, Little MP, McHugh K, Lee C, Kim KP, et al. Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study. Lancet. 2012;380(9840):499-505. 52. Huang WY, Muo CH, Lin CY, Jen YM, Yang MH, Lin JC, et al. Paediatric head CT scan and subsequent risk of malignancy and benign brain tumour: a nation-wide population-based cohort study. Br J Cancer. 2014;110(9):2354-60. 53. Krille L, Dreger S, Schindel R, Albrecht T, Asmussen M, Barkhausen J, et al. Risk of cancer incidence before the age of 15 years after exposure to ionising radiation from computed tomography: results from a German cohort study. Radiat Environ Biophys. 2015;54(1):1-12. 54. Pauwels R, Cockmartin L, Ivanauskaite D, Urboniene A, Gavala S, Donta C, et al. Estimating cancer risk from dental cone-beam CT exposures based on skin dosimetry. Phys Med Biol. 2014;59(14):3877-91. 55. Aanenson JW, Till JE, Grogan HA. Understanding and communicating radiation dose and risk from cone beam computed tomography in dentistry. J Prosthet Dent. 2018. 56. Yeh JK, Chen CH. Estimated radiation risk of cancer from dental cone-beam computed tomography imaging in orthodontics patients. BMC Oral Health. 2018;18(1):131. 57. Brenner DJ. Estimating cancer risks from pediatric CT: going from the qualitative to the quantitative. Pediatr Radiol. 2002;32(4):228-1; discussion 42-4. 58. Hall EJ. Lessons we have learned from our children: cancer risks from diagnostic radiology. Pediatr Radiol. 2002;32(10):700-6. 59. Schroeder AR, Redberg RF. The harm in looking. JAMA Pediatr. 2013;167(8):693-5. 60. De Grauwe A, Ayaz I, Shujaat S, Dimitrov S, Gbadegbegnon L, Vande Vannet B, et al. CBCT in orthodontics: a systematic review on justification of CBCT in a paediatric population prior to orthodontic treatment. Eur J Orthod. 2018. 61. Bogdanich W. CMJ. Radiation Worries for Children in Dentists’ Chairs. New York Times. 2010. 62. Gee A. Radiation Concerns Rise With Patients’ Exposure. New York Times. 2012 June 13 2012. 63. Ruhm W, Eidemuller M, Kaiser JC. Biologically-based mechanistic models of radiation-related carcinogenesis applied to epidemiological data. Int J Radiat Biol. 2017;93(10):1093-117. 64. Gronthos S, Mankani M, Brahim J, Robey PG, Shi S. Postnatal human dental pulp
stem cells (DPSCs) in vitro and in vivo. Proc Natl Acad Sci U S A. 2000;97(25):13625-30. 65. Miura M, Gronthos S, Zhao M, Lu B, Fisher LW, Robey PG, et al. SHED: stem cells from human exfoliated deciduous teeth. Proc Natl Acad Sci U S A. 2003;100(10):5807-12. 66. Morsczeck C, Gotz W, Schierholz J, Zeilhofer F, Kuhn U, Mohl C, et al. Isolation of precursor cells (PCs) from human dental follicle of wisdom teeth. Matrix Biol. 2005;24(2):155-65. 67. Sonoyama W, Liu Y, Yamaza T, Tuan RS, Wang S, Shi S, et al. Characterization of the apical papilla and its residing stem cells from human immature permanent teeth: a pilot study. J Endod. 2008;34(2):166-71. 68. Seo BM, Miura M, Gronthos S, Bartold PM, Batouli S, Brahim J, et al. Investigation of multipotent postnatal stem cells from human periodontal ligament. Lancet. 2004;364(9429):149-55.
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69. Thomas P, Holland N, Bolognesi C, Kirsch-Volders M, Bonassi S, Zeiger E, et al. Buccal
micronucleus cytome assay. Nat Protoc. 2009;4(6):825-37. 70. Ozkul Y, Donmez H, Erenmemisoglu A, Demirtas H, Imamoglu N. Induction of micronuclei by smokeless tobacco on buccal mucosa cells of habitual users. Mutagenesis. 1997;12(4):285-7. 71. Kashyap B, Reddy PS. Micronuclei assay of exfoliated oral buccal cells: means to assess the nuclear abnormalities in different diseases. J Cancer Res Ther. 2012;8(2):184-91. 72. Gonzalez JE, Roch-Lefevre SH, Mandina T, Garcia O, Roy L. Induction of gamma-H2AX foci in human exfoliated buccal cells after in vitro exposure to ionising radiation. Int J Radiat Biol. 2010;86(9):752-9. 73. Siddiqui MS, Francois M, Fenech MF, Leifert WR. gammaH2AX responses in human buccal cells exposed to ionizing radiation. Cytometry A. 2015;87(4):296-308. 74. Humphrey SP, Williamson RT. A review of saliva: normal composition, flow, and function. J Prosthet Dent. 2001;85(2):162-9. 75. Pernot E, Cardis E, Badie C. Usefulness of saliva samples for biomarker studies in radiation research. Cancer Epidemiol Biomarkers Prev. 2014;23(12):2673-80. 76. Hassaneen M, Maron JL. Salivary Diagnostics in Pediatrics: Applicability, Translatability, and Limitations. Front Public Health. 2017;5:83. 77. Farnaud SJ, Kosti O, Getting SJ, Renshaw D. Saliva: physiology and diagnostic potential in health and disease. ScientificWorldJournal. 2010;10:434-56. 78. Moore HD, Ivey RG, Voytovich UJ, Lin C, Stirewalt DL, Pogosova-Agadjanyan EL, et al. The human salivary proteome is radiation responsive. Radiat Res. 2014;181(5):521-30.
Chapter 3:Method validation to assess in vivo cellular and subcellular changes in buccal mucosa cells and
saliva following CBCT examinations
63
Chapter 3:
Method validation to assess in
vivo cellular and subcellular
changes in buccal mucosa cells
and saliva following CBCT
examinations
Belmans N, Gilles L, Virag P, Hedesiu M, Salmon B, Baatout S, Lucas S, Jacobs
R, Lambrichts I, and Moreels M (2019) Method validation to assess in vivo cellular
and subcellular changes in buccal mucosa cells and saliva following CBCT
examinations. Dentomaxillofacial Radiology – Published online April 5th, 2019 -
doi:10.1259/dmfr.20180428
Chapter 3:Method validation to assess in vivo cellular and subcellular changes in buccal mucosa cells and
saliva following CBCT examinations
65
3.1 Abstract
Objectives
Cone-beam computed tomography (CBCT) is a medical imaging technique
used in dental medicine. However, there are no conclusive data available
indicating that exposure to X-ray doses used by CBCT are harmless. We aim, for
the first time, to characterize the potential age-dependent cellular and subcellular
effects related to exposure to CBCT imaging. Current objective is to describe and
validate the protocol for characterization of cellular and subcellular changes after
diagnostic CBCT.
Methods
Development and validation of a dedicated two-part protocol: 1) assessing
DNA double strand breaks (DSBs) in buccal mucosal (BM) cells and 2) oxidative
stress measurements in saliva samples. BM cells and saliva samples are collected
prior to and 0.5 hours after CBCT examination. BM cells are also collected 24 hours
after CBCT examination. DNA DSBs are monitored in BM cells via
immunocytochemical staining for γH2AX and 53BP1. 8-oxo-7,8-dihydro-2’-
deoxyguanosine (8-oxo-dG) and total antioxidant capacity are measured in saliva
to assess oxidative damage.
Results
Validation experiments show that sufficient BM cells are collected (97.1% ±
1.4%) and that γH2AX/53BP1 foci can be detected before and after CBCT
examination. Collection and analysis of saliva samples, either sham exposed or
exposed to IR, show that changes in 8-oxo-dG and total antioxidant capacity can
be detected in saliva samples after CBCT examination.
Conclusion
The DIMITRA Research Group presents a two-part protocol to analyse
potential age-related biological differences following CBCT examinations. This
protocol was validated for collecting BM cells and saliva and for analysing these
samples for DNA DSBs and oxidative stress markers, respectively.
Keywords:
Dental Cone-Beam Computed Tomography – DNA Double Strand Breaks –
Oxidative stress – Buccal mucosal cells - Saliva
Chapter 3:Method validation to assess in vivo cellular and subcellular changes in buccal mucosa cells and
saliva following CBCT examinations
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3.2 Introduction
Dental cone-beam computed tomography (CBCT) is a relatively new and
innovative diagnostic imaging technique introduced in oral health care at the turn
of the century.(1, 2) Its growing use lies in the diagnostic potential related to the
transition from two-dimensional (2D) to three-dimensional (3D)
dentomaxillofacial diagnostic imaging.(3-6) CBCT uses a cone-shaped X-ray beam
and a 2D detector to generate 3D images. Briefly, the source-detector rotates
around the patient once, while generating a series of 2D images. These images
are then reconstructed into a 3D volume data set using a specialized algorithm.(3,
7-9) Specifically designed to produce cross-sectional images of the oral and
maxillofacial region, combined with its low cost and easy accessibility, CBCT
technology has rapidly evolved in the past decade. Nowadays it has become a
widely available diagnostic tool for clinicians and has therefore found applications
in multiple dental specialties, including implant planning, endodontics,
orthodontics and maxillofacial surgery.(1, 2, 4, 8, 10-12)
Like other medical imaging techniques, such as computed tomography (CT),
CBCT uses X-rays for its image acquisition. However, ionizing radiation (IR) is
capable of damaging biomolecules (e.g. DNA or proteins) directly or indirectly via
the hydrolysis of water which generates free radicals, such as reactive oxygen
species (ROS).(13, 14) Although CBCT is defined as a low dose imaging technique
by the European High-Level Expert Group on European Low Dose Risk Research
(HLEG) (www.hleg.de), it is misleading to see it as a ‘low-dose’ imaging modality
just because it only takes one rotation compared to multiple rotations in
conventional CT. As in CT, the absorbed dose in CBCT heavily depends on
selectable exposure parameters that determine the image quality such as kVp,
mAs, field of view (FOV), amount of 2D projections, reconstitution algorithm,
etc..(4, 15-18) Therefore, a wide range of CBCT doses is observed, typically ranging
from about 0.010 to 1.100 mSv per examination.(15, 17-22) CBCT doses are lower
than CT doses (organ dose of about 15 mSv), however, they are higher than
classical 2D dental radiography techniques (organ dose of 0.001 – 0.1 mSv).(4, 16,
23-26)
More recently, the dose of ionizing radiation delivered to pediatric patients
has become a major concern among clinicians worldwide.(20, 24) In 2010, the New
York Times was the first major newspaper to bring this concern to the attention
of the general public when they published the article entitled “Radiation Worries
for Children in Dentists’ Chairs”.(27) In practice, especially in orthodontics, a large
portion of CBCT examinations is performed on children (< 18 years old), who are
known to be more radiosensitive than adults.(18, 28-30) These concerns about the
dose, combined with an increasing amount of radiological examinations annually,
Chapter 3:Method validation to assess in vivo cellular and subcellular changes in buccal mucosa cells and
saliva following CBCT examinations
67
have led to questions about the biological uncertainties associated with radiation-
induced health risks at low doses in dental radiology.(24, 31, 32)
Exposure to IR, such as X-rays, could result in damage to important
biomolecules, either directly, but mostly indirectly via generation of free radicals,
usually through hydrolysis of water. These radicals (e.g. reactive oxygen species
(ROS)) can in turn damage biomolecules in nano- to microseconds.(14) Since more
than 60% of a cell consists of water, most of the DNA damage is caused indirectly
via ROS (e.g. the hydroxyl radical, superoxide radicals and hydrogen peroxide).(25,
33) An excess of ROS causes oxidative stress. In the context of oral pathology,
oxidative stress is associated with periodontitis, dental caries and oral cancers.(34,
35) ROS can cause oxidative DNA damage through oxidative base lesions, of which
over 20 different lesions have been identified.(36) An example hereof is 8-oxo-7,8-
dihydro-2’-deoxyguanosine (8-oxo-dG), a mutagenic base modification.(37) Other
types of DNA lesions include single strand breaks, double strand breaks (DSBs)
and base alterations.(33, 38) DNA double strand breaks (DSBs) are the most critical
DNA lesions caused by IR. When not repaired correctly, DSBs can lead to
chromosome rearrangements, mutations and loss of genetic information.(39-44) To
protect themselves, eukaryotic cells have developed the DNA damage response
(DDR), a set of signalling and DNA repair pathways.(45-47)
Human buccal mucosa (BM) cells are useful for determining exposure to
several environmental factors.(48, 49) Furthermore, BM cells are an easy accessible
source of cells that can be sampled in a minimally invasive way.(50, 51) As such,
they are being increasingly used to investigate the effects of exposure to
genotoxins that can cause DNA damage and cell death.(48, 51, 52)
Another easy accessible biological sample is saliva, which, like BM cells, is
easy to collect in an inexpensive, painless and non-invasive way.(53) Known as the
‘mirror of the body’, saliva is finding its way to research and the clinic as a
diagnostic fluid.(35, 54, 55) To date, the salivary metabolome has been described and
saliva has been used to link oxidative stress markers to several oral diseases,
such as dental caries and periodontitis.(34, 35, 56)
Effective dose (ED), measured in mSv, is a dose quantity that takes
following factors into account: 1) the absorbed dose to all organs of the body, 2)
the relative harm of the type of radiation, and 3) the radiosensitivity of each
organ. Although ED is an accepted term since its introduction in radiation
protection, it is often criticized. For example the weighing factors used to calculate
the ED are determined by scientific committees and may evolve over time.(57-59)
Furthermore, the ED is independent of gender and age at exposure, whereas
epidemiological data indicate that both gender and age at exposure are important
parameters.(60)
A European project funded by the Open Project for European Radiation
Research Area (OPERRA) denoted as DIMITRA (Dentomaxillofacial Paediatric
Imaging: An Investigation Towards Low Dose Radiation Induced Risks) was
Chapter 3:Method validation to assess in vivo cellular and subcellular changes in buccal mucosa cells and
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68
initiated in order to characterize any potential cellular and subcellular effects
induced by dental CBCT imaging, with a focus on age- and gender specificity and
with reference to simulated ED (www.dimitra.be). In vitro results from DIMITRA
were published previously, showing transient increases in DNA DSBs and changes
in inflammatory cytokines after CBCT exposure of dental stem cells in vitro.(61)
The objective of the present report is to describe and validate a two-part protocol
enabling the DIMITRA project to assess the potential age-related cellular and
subcellular effects using DNA DSB detection in buccal mucosal cells and salivary
oxidative stress measurement. To the best of our knowledge, a protocol and
method validation for characterizing cellular and subcellular effects of CBCT
exposure has not yet been described.
Chapter 3:Method validation to assess in vivo cellular and subcellular changes in buccal mucosa cells and
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69
3.3 Materials and methods
3.3.1 Description of the DIMITRA protocol
Synthetic swabs (EpiCentre®, Madison, USA) are used to collect BM cells
from eligible patients. Eligibility criteria are: having no systemic or acute diseases,
taking no medication (antibiotics or anti-inflammatory drugs), having a good oral
hygiene and giving informed consent prior to conclusion. When eligible, patients
were asked to complete a questionnaire (supplementary data 1). At least one hour
prior to BM cell collection, subjects are asked not to eat, brush their teeth or
smoke. Just before BM cell collection, subjects rinse their mouth twice with water
to remove excess debris. BM cells are collected from each patient just before, 0.5
hours after and 24 hours after CBCT examination (figure 3.1), using a protocol
modified from Thomas et al. (2009).(50) The 24 hours samples are collected at the
patients’ homes. To this end patients receive detailed instruction sheets
(supplementary data 2). After collection, samples are sent to SCK•CEN via a
professional courier service.
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Figure 3.1. Flow chart for patient inclusion and patient sampling. CBCT = Cone
Beam Computed Tomography; BM = Buccal mucosa
3.3.2 Buccal mucosal cell collection and fixation
Per patient six 15 ml conical tubes (Cellstar®, Greiner Bio-One, Vilvoorde,
Belgium) (one for each time point and cheek side) containing 10 ml of
Saccomanno’s fixative (SF) (50% ethanol, 2% polyethylene glycol, 48% MilliQ
water) are prepared. The swab is taken out of the package by the plastic handle.
It is important not to touch the swab itself. Then the swab is placed against the
middle of the patient’s cheek. For reproducibility, the same cheek was used every
time. Next, it is pressed firmly against the cheek and moved in an upward-
downward motion while turning the swab for at least 30 seconds. The swab is then
placed into SF in the 15 ml conical tube and shaken in such a manner that the
cells are dislodged and released into SF. The tubes are then stored at 4°C (for up
to 7 days) before shipment to SCK•CEN by courier service.
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Within 7 days after sample collection, the BM cells are harvested from SF.
For this purpose, the 15 ml conical tubes are centrifuged at 580g for 10 minutes
at room temperature (RT). The supernatant is aspirated until about 1 ml is left. 5
ml of autoclaved buccal buffer (BuBu) (0.01 M Tris-HCl, 0.1 M EDTA, 0.02 M NaCl,
1% FBS, pH = 7) is added to the tube, after which the cells are vortexed briefly.
Then, the cells are centrifuged at 580g for 10 minutes at RT. The supernatant is
removed completely and the cells are washed with 5 ml BuBu and centrifuged at
580g for 10 minutes at RT. This washing step is repeated twice to inactivate
DNAses from the oral cavity and to remove excess debris and bacteria. After
washing, the supernatant is removed and the cells are resuspended in 5 ml of
BuBu and vortexed briefly. Next, the BM cells are passed through a 100 µm nylon
filter (Falcon®, VWR Belgium, Leuven, Belgium) into a 50 ml conical tube
(Cellstar®, Greiner Bio-One, Vilvoorde, Belgium) to remove large aggregates of
unseparated cells. The 50 ml conical tube holding the filter is then centrifuged at
580g for 10 minutes at RT. Afterwards, the BM cells in the filtrate are transferred
to a new 15 ml conical tube. Then the BM cells are centrifuged one last time at
580g for 5 minutes at RT. The supernatant is removed and the BM cells are
resuspended in 1 ml of BuBu. The BM cells are then centrifuged at 580g for 5
minutes at RT and the supernatant is discarded afterwards. Then, the BM cells are
fixed in 500 µl of 2% paraformaldehyde (PFA) (Sigma Aldrich, St-Louis, MO, USA)
while vortexing the BM cells and adding the PFA dropwise. The BM cells are
incubated for at least 15 minutes at RT. After incubation, the BM cells are
centrifuged at 580g for 5 minutes. The supernatant is discarded and the BM cells
are washed twice using 1x phosphate-buffered saline (PBS) (Gibco, Life
Technologies, Ghent, Belgium). After the last washing step, the BM cells are
resuspended in 1 ml 1x PBS. The BM cells can now be stored at 4°C for a longer
period or used immediately for immunocytochemical staining.
3.3.3 Immunocytological staining for DNA double strand breaks: γH2AX
and 53BP1 staining
Before immunocytochemical staining, the BM cells need to be transferred
from the 15 ml conical tubes to coverslips by cytocentrifugation. The BM cells are
washed using 200 µl of 1x PBS twice. During washing, poly-L-lysine coated
coverslips, which assure good attachment of the BM cells, are placed on a
microscope slide which is then inserted in a cytofunnel (ThermoFisher, Waltham,
MA, USA). Next, 100 µl of cell suspension is pipetted into each sample cup of a
Cytofunnel. The cytofunnels are centrifuged at 1200 rpm for 10 minutes in a
cytocentrifuge (ThermoFisher, Waltham, MA, USA) at RT, causing the BM cells to
adhere to the coverslip inside the cytofunnel. After centrifugation, the coverslips
are removed and placed into a 4-well culture plate (Nunc, ThermoFisher Scientific,
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Roskilde, Denmark) so the BM cells are facing up. The BM cells are allowed to air-
dry for 2 minutes at RT.
Immunocytochemical staining was performed using a protocol as previously
described by our group.(62-64) First the BM cells are washed twice using cold 1x
PBS for 5 minutes on a rocking platform. After washing, the BM cells are
permeabilized for 3 minutes using 0.25% Triton X-100 in 1x PBS at RT. Next, the
BM cells are washed three times with 1x PBS. Then the BM cells are blocked with
1x pre-immunized goat serum (ThermoFisher Scientific, Waltham, MA USA) in a
solution of 1x TBST, 0.005 g/v% TSA blocking powder (PerkinElmer, FP1012,
Zaventem, Belgium) (TNB) for 1 hour at RT. After blocking the primary mouse
monoclonal anti-γH2AX antibody (Millipore 05-636, Merck, Overijse, Belgium)
(1:300 in TNB) and rabbit polyclonal anti-53BP1 antibody (Novus Biologicals
NB100-304, Abingdon, UK) (1:1000 in TNB) are added. Next, the BM cells are
incubated overnight at 4°C on a rocking platform. After incubation, the BM cells
are washed three times with 1x PBS. Then the secondary goat anti-mouse Alexa
Fluor® 488-labeled antibody (1:300 in TNB) and goat anti-rabbit Alexa Fluor®
568-labeld antibody (1:1000 in TNB) (ThermoFisher Scientific, A11001, Waltham,
MA USA) were added. The BM cells are incubated for 1 hour on a rocking platform
in the dark. Afterwards, the BM cells are washed twice using 1x PBS. Next, slides
are mounted with ProLong Diamond antifade medium with 4',6-diamidino-2-
phenylindole (DAPI) (ThermoFisher Scientific, Waltham, MA USA).
Finally, images are acquired with a Nikon Eclipse Ti fluorescence microscope
using a 40× dry objective (Nikon, Tokyo, Japan). Images are analyzed using open
source Fiji software.(65) The software allows to analyze each nucleus based on the
DAPI signal. Within each nucleus, the intensity signals from the Alexa 488 and
Alexa 568 fluorochromes are analyzed after which the number of co-localized
γH2AX and 53BP1 foci per nucleus are determined in an automated manner using
the Cellblocks toolbox (figure 3.2).(66)
3.3.4 Saliva collection and analysis
Saliva samples are collected right before and 0.5 hours after CBCT
examination (figure 3.1) using the passive drool method, which is considered to
be the ‘gold standard’ for saliva sampling.(67) As with the BM cells (saliva is
sampled at the same time), subjects are asked not to eat, brush their teeth or
smoke one hour prior to saliva sampling. Just before saliva collection, subjects
will rinse their mouth twice with water to remove excess debris. If blood is
detected in the saliva, the sample is not included for this study. The saliva samples
will be stored at -20°C immediately after collection before shipment to SCK•CEN
by courier service. Once at SCK•CEN samples will be centrifuged at 10 000g at
4°C to remove most of the mucus and the supernatant will be stored at -80°C.
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The stored samples will be used to determine 8-oxo-dG concentrations and the
total antioxidant capacity (figure 3.2).
3.3.5 8-oxo-dG determination
8-oxo-dG concentrations will be determined by competitive enzyme-linked
immunosorbent assay (ELISA) (Health Biomarkers Sweden AB, Stockholm,
Sweden). To remove substances other than 8-oxo-dG which could cross-react with
the monoclonal antibody used in the ELISA-kit, 800 µL sample will be purified
prior to ELISA using a C18 solid phase extraction column (Varian, Lake Forest,
CA, USA) after which the samples are freeze-dried. This purification is performed
twice.(68)
The 8-oxo-dG concentration of saliva will be measured based on a modified
ELISA protocol provided by Health Biomarkers Sweden AB (Stockholm, Sweden).
The protocol will be performed as previously described by Haghdoost et al..(69)
Briefly, 270 µl of purified sample/standard will be mixed with 165 µl of primary
antibody (80 ng/ml) mix in Eppendorf tubes. Next the samples will be incubated
for 2 hours at 37°C. During incubation, the ELISA plate will be washed twice using
1x PBS. After incubation 140 µl of sample/standard will be loaded onto the plate
in triplicate. The plate will be incubated overnight at 4°C on a horizontal shaker.
Next the plate will be washed three times using 1x washing solution. After washing
140 µl of secondary antibody mix is added to each well. The plate is incubated for
2 hours at RT on a horizontal shaker. Next the plate is washed three times with
1x washing solution and once more with 1x PBS. Finally, the reaction is visualized
by the addition of 140 μl chromogenic substrate 3,3',5,5'-Tetramethylbenzidine
(One-Step substrate system; Dako, Glostrup Municipality, Denmark), and further
incubation in the dark for 15 minutes. The reaction is stopped by adding 70 μl of
2M H2SO4. The absorbance is measured at 450 nm (signal) and 570 nm
(background) using a microplate reader (ClarioStar, BMG Labtech, Ortenberg,
Germany) (figure 3.2).
3.3.6 Total antioxidant capacity
To determine the antioxidant capacity of saliva samples, the ferric reducing
antioxidant power (FRAP) assay is used (Cell Biolabs, CA, USA). The FRAP assay
will be performed according to the manufacturer’s instructions. Briefly, per well of
a 96-well plate 100 µl of sample/standard and 100 µl of reaction reagent are
added. Next the samples/standards are incubated for 10 minutes at RT on a
horizontal shaker. Finally, the absorbance will be measured at 560 nm using a
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microplate reader (ClarioStar, BMG Labtech, Ortenberg, Germany). The results
will be expressed as Iron(II) concentration (µM) or FRAP value (figure 3.2).
Figure 3.2. Flow chart for sample analysis. Schematic view of DNA double strand
break detection in buccal mucosal cells and oxidative stress measurements in saliva
samples. DSB = Double-strand break; BM = Buccal mucosa; γH2AX = phosphorylated
histone 2AX on Ser139; 53BP1 = p53-binding protein 1; 8-oxo-dG = 8-oxo-7,8-dihydro-
2’-deoxyguanosine; FRAP = Ferric Reducing Antioxidant Power; ELISA = Enzyme-linked
Immunosorbent assay
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3.4 Protocol validation
3.4.1 Pilot study population
Healthy adults (N = 6) are included in this pilot study to validate the
DIMITRA study protocol. These patients are referred for a CBCT examination. All
patients were asked to sign informed consent forms prior to being included in the
study. The validation study was approved by the ethical committees of the
participating hospitals, since this is part of the scope of the DIMITRA study.
3.4.2 Flow cytometrical identification of buccal mucosal cells
Cells collected using the method described earlier are identified with the
epithelial cell marker cytokeratin 4 (CK4) and lymphoid cell marker CD45 to
identify the amount of BM cells collected with the swab. A431 and PC3 (courtesy
of Katrien Konings) cell lines are used as a positive control for CK4 expression.
Jurkat cells are used as a positive control for CD45 expression.
All cells are washed with 1xPBS and fixed in ice-cold (-20°C) 70% ethanol
at a concentration of 1x106 cells/ml or 2x106 cells/ml (Jurkat). Next, cells are
washed once with a solution of 1x PBS, 5% FBS (GIBCO, Life Technologies, Ghent,
Belgium) and 0.25% Triton X-100 (Sigma-Aldrich chemistry, St-Louis, MO USA)
(PFT) and are then blocked for 1h at RT in PFT. After blocking, cells are incubated
with a rabbit anti-CK4 antibody (diluted 1:100 in PFT) overnight at 4°C on a
horizontal shaker. Next, cells are washed twice with PFT. Subsequently, Alexa
488-conjugated donkey anti-rabbit secondary antibody (diluted 1:200 in PFT) and
primary mouse anti-human CD45 antibody labelled with allophycocyanin (diluted
1:50 in PFT) are added and the cells were incubated for 2h at RT in the dark. After
incubation, the cells are washed twice with PFT and treated with 10 µg/ml of the
DNA dye 7-AminoActinomycin D (7-AAD) for 15 min at RT. 7-AAD is used to
distinguish cellular material from debris. Furthermore, it gives information about
the current cell cycle phase of the samples. Finally, the samples are filtered on a
BD conical tube (Falcon ®, Corning, NY, USA) and analyzed on the BD AccuriTM
C6 Flow Cytometer (BD Biosciences, San Jose, CA USA). At least 10.000 events
are measured. Single-colour stained cells are included for colour compensation.
Gating is based on using A431, PC3 and Jurkat cells as positive/negative control
for CK4 or CD45. Cells in G1/G0 phase and CK4+ are identified as BM cells.
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3.4.3 Histological staining for epithelial cell identification
Cells are collected using the method described earlier and were stained
using Giemsa to allow for histological examination of the cells collected in the
swab. After the cells are fixed in 2% PFA, they are spotted on poly-L-lysine coated
coverslips (see above). Next, the cells are stained with Giemsa (1:50 in 0.2M
acetate buffer, pH = 3.36) (VWR International, Radnor, PA, USA) for 1 hour at
RT. After incubation, the cells are washed twice with milliQ water. Next, the slides
are mounted with DPX (VWR International, Radnor, PA, USA). Finally, images are
acquired with a Nikon Eclipse Ti microscope using a 20× dry objective for
brightfield image acquisition (Nikon, Tokyo, Japan).
3.4.4 Statistics
Statistical analyses is performed using GraphPad Prism 7.02 (GraphPad
Inc., CA, USA). Induction of DNA DSBs in BM cells is analysed using repeated
measures ANOVA. Both 8-oxo-dG concentrations and FRAP values before and after
CBCT are compared using a paired t-test. To perform the above listed parametric
tests, values should be normally distributed and the variances should be equal.
Should these conditions not be met, non-parametric alternatives are used. P
values lower than 0.05 are considered as statistically significant. Age-related
effects are not considered during the validation experiment.
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3.5 Results
Validation of the described protocol was performed on samples collected
from adults (Table 3.1). BM cells were collected from adult volunteers (n = 6)
using buccal swabs. Characterization of the cells collected by the swabs was
performed using flow cytometrical and light microscopical analysis. CK4+ cells
(that were in G1/G0 phase) were identified as BM cells. Flow cytometrical analysis
showed that 97.1% ± 1.4% of the cells were CK4+ BM cells, whereas less than
1% of cells were CD45+. These CD45+ cells are most likely leukocytes (figure 3.3).
Further histological analysis confirmed that the collected cells are indeed BM cells,
in various stages of exfoliation: some are nucleated, while others are not (figure
3.4A, arrowheads).
Table 3.1. Overview of scan parameters per patient included in this validation
study.
Patient Age Sex Device Field of
view
mAs kV Acquisition time
(seconds)
1 57 Female Newtom
VGi evo 10x5 11 110 5
2 41 Female Newtom
VGi evo 10x5 6 110 5
3 30 Female Newtom
VGi evo 10x10 8 110 5
4 30 Male Newtom
VGi evo 10x10 10 110 5
5 71 Male Newtom
VGi evo 10x10 8 110 5
6 27 Female Newtom
VGi evo 10x10 8 110 5
mAs = milliamperage; kV = kilovoltage
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The presence of DNA DSBs in BM cells was detected using an
immunocytochemical staining for γH2AX and 53BP1 (figure 3.4B-E). Analysis of
colocalized γH2AX and 53BP1 foci shows that 0.015 ± 0.012 foci/nuclei were
counted before CBCT and 0.028 ± 0.028 foci/nuclei were counted after (p = 0.99).
Saliva samples were collected from adults that were subjected to CBCT
examination twice: once without IR exposure (sham control = group 1) and once
with IR exposure (= group 2). These samples (n = 5) were used to validate the
protocols for the 8-oxo-dG and FRAP determination.
The change in 8-oxo-dG levels before and after CBCT exposure between
group 1 and group 2 was compared. Group 1 showed no difference (-0.09 ± 0.44
ng/ml; p = 0.88) in 8-oxo-dG levels whereas an increasing trend was found in
group 2 (2.5 ± 3.0 ng/ml; p = 0.19). Comparison of the changes in both groups
was not significant (p = 0.15), but it shows that after IR exposure (due to CBCT
examination) changes in 8-oxo-dG levels can be detected.
In combination with the 8-oxo-dG ELISA, a FRAP assay was performed.
When comparing FRAP values before and after CBCT examination, results show
that the FRAP value does not change in group 1 (-3.6 ± 69; p > 0.99), but there
is a decreasing trend in group 2 (-18 ± 49; p = 0.31). The change between both
groups does not differ significantly (p = 0.89), but these data show that after IR
exposure (due to CBCT examination) changes in FRAP values can be detected.
Figure 3.3. Flow cytometrical identification of cells collected by buccal swab. A.
Overview of the cells that were in G1/G0 phase. Note that no S or G2/M phase were observed,
indicating that the cells are fully differentiated cells. B. Over 97% of the cells collected by
buccal swab are CK4+ epithelial cells (= buccal cells), whereas less than 1% are CD45+,
indicating that cells of hematological lineage are present (N = 6).
Chapter 3:Method validation to assess in vivo cellular and subcellular changes in buccal mucosa cells and
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Figure 3.4. Microscopical identification of cells collected by buccal swab. A. Giemsa
stain clearly shows nucleated epithelial cells (arrowheads), as well as unnucleated cells. This
indicates that cells from all mucosal layers are collected. Enough nucleated cells are collected
to perform immunocytochemistry. B-E. Buccal cells with DNA double strand break identified
by colocolization of γH2AX and 53BP1. B. Buccal cell nucleus, DAPI stain. C. γH2AX-positive
focus. D. 53BP1-positive focus. E. Merged image of B, D and E.
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3.6 Discussion
Currently, the main challenge in the field of radiation protection is
identifying biomarkers that allow detection of cellular and subcellular changes due
to exposure to low doses of IR (< 0.1 Gy). These biomarkers could then be used
to predict low dose IR-associated risks. To this end, blood is the most commonly
used sample to study cellular and subcellular changes in the low dose range, such
as the doses used in medical diagnostic imaging. Blood contains numerous cells
that can be used for a variety of assays used in low dose radiation research, such
as the micronucleus assay, dicentric assay, comet assay, γH2AX assay, oxidative
stress tests (e.g. 8-oxo-dG) and even gene expression assays.(70-76) The
advantage of blood sampling is that a standardized protocol can be used, the
procedure is easy and small volumes suffice for most tests performed. However,
the major limitation of drawing blood is that the procedure is invasive, which can
cause discomfort to the patient, especially to pediatric patients.(70)
The DIMITRA Research Group provides a two-part protocol to assess
potential cellular and subcellular effects after exposure to low doses of IR, i.e.
CBCT examinations. This protocol focusses on non-invasive samples, i.e. BM cells
and saliva samples. Compared to blood samples, BM cells and saliva samples have
several major advantages: collection is non-invasive, cheap, painless and
therefore allows easy repeated sampling.(50, 51, 53) This opens new opportunities
for use in (oral) healthcare with an increased suitability when pediatric patients
are involved. The two-part protocol focusses on detection of DNA DSBs and
oxidative stress markers. Oxidative stress can induce oxidative DNA damage
which has mutagenic and tumorigenic potential.(77) DNA DSBs, which can (partly)
be caused by oxidative stress, is associated with carcinogenesis, an important
health risk related to IR exposure.(78, 79) Therefore, DNA DSB formation and repair
are important markers to assess potential health risks in patients exposed to IR.
The current paper describes and validates this two-part protocol. The
collection method for BM cells was validated by flow cytometry (presence of G1/G0
phase CK4+ cells) and light microscopy (Giemsa staining). BM cells from different
mucosal layers were collected, although the majority of the cells were nucleated.
These results show that this collection method yields sufficient BM cells for
microscopical analysis. The use of γH2AX foci in BM cells is described before as is
the use of a γH2AX/53BP1 immunofluorescent staining for the detection of DNA
DSBs.(51, 64, 80-82) However, to the best of our knowledge, this is the first time that
a protocol is proposed to detect DNA DSBs after CBCT examination, although other
genotoxicity markers have been published before.(83) Our validation data show
that that ex vivo BM cells can be used to perform γH2AX/53BP1 analysis. Future
studies will investigate whether age-dependent differences can be detected in the
amount of DNA DSBs after CBCT examination. For saliva collection, a protocol was
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described based on the passive drool method, after which the samples are
immediately stored at -20°C. Comparison between sham exposure and IR
exposure, i.e. CBCT examination, shows that changes in 8-oxo-dG and FRAP levels
can be detected in saliva samples after CBCT examination. These findings confirm
that the methods described in this paper are suited for evaluating potential effects
of low dose IR exposure in BM cells and saliva samples. The changes detected
here are small, but can be attributed to the age of the volunteers: adults are more
radioresistant than children, therefore we hypothesize that the effects of low dose
IR exposure might be greater in children.
Despite the aforementioned advantages and validation of the DIMITRA
study protocol, some precautions should be taken into account when using BM
cells and saliva. BM consists of several layers of cells, thus sampling should be
done in an uniformed way to avoid differences in cell type distribution. For
example, it is known that the amount of basal cells increases when the cheek is
sampled repeatedly.(48, 50) Therefore, the authors suggest to collect some test
samples prior to the actual study and to characterize the cells that are collected,
as described earlier. Although cigarette/cigar smoke is a known cytotoxin and
genotoxin to BM cells(84), one limitation of this validation protocol is that ‘smoking’
was not included in the exclusion criteria. Therefore, it is recommended to add
‘smoking’ as an exclusion criterion when conducting studies in which BM cells are
collected for this type of study.
Saliva composition can be affected by several factors, such as the collection
itself, time of day, intake of antioxidants, time since tooth-brushing, presence of
blood, drug intake, etc.. Moreover, some (pediatric) patients might not be able to
produce (enough) saliva spontaneously. However, the authors recommend to not
induce salivation actively, since this will create a bias when compared with
spontaneous salivation.(35) To keep this type of bias to a minimum, our protocol
is based on the passive drooling method to collect saliva, which is regarded as the
gold standard.(67) Additional information from the patients on drug intake,
previous radiation exposure, etc. should be obtained as well through a
questionnaire.
For the post-imaging assessment, 30 minutes and 24 hours were chosen
for γH2AX/53BP1 staining based on previous results from SCK•CEN, in which the
peak response is seen after 30 to 60 minutes and most DNA damage is resolved
after 24 hours.(62-64) For the 8-oxo-dG analysis and FRAP assay, we chose time
points based on Haghdoost et al., who tested 8-oxo-dG after 30 minutes.(69) This
coincides with BM cell sampling, which is an advantage since this way DNA DSB
and 8-oxo-dG levels can be correlated. The results show that changes, especially
in oxidative stress markers, can be detected at this time. However, it is possible
that the selected time points are not the most optimal ones. Finally, we are not
certain that the described methods for detecting DNA damage will be sensitive
enough to detect changes following CBCT examination in children, since to the
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best of the authors’ knowledge, this type of study has not been performed before.
Current time points are selected based on literature, as mentioned above, but also
out of practical consideration: i.e. not letting the patient wait too long after the
CBCT examination. If necessary, and if patients are willing, it may be possible to
include additional time points (e.g. 60 minutes after CBCT examination).
The DIMITRA study protocol presented here is designed to be cost effective,
quick, painless and non-invasive. The use of this protocol, however, is not limited
to this study and can be easily implemented in other (radio)biological studies. For
example, this protocol can be used in a similar setting in which patients are
exposed to a head and neck CT, or in cancer patients treated for head and neck
cancer. Furthermore, the use of saliva can be used to monitor patients exposed
to short- and long-lived radionuclides for diagnostics/therapy. These examples
expand the use of this protocol from risk assessment in medical diagnostics, to
follow-up/monitoring of radiotherapy patients, two distinctive field in medicine
using ionizing radiation.
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3.7 Conclusion
It is well-known that children are more radiosensitive than adults. Together
with the increasing amount of radiological examinations annually, this has recently
led to societal concerns about exposure to IR during medical procedures. The
DIMITRA Research Group presents a dedicated, two-part protocol to analyse
potential age-related biological differences in response to CBCT examinations in
both pediatric and adult patients. This protocol was validated for collecting BM
cells and saliva, as well as for analysing BM cells and saliva samples for DNA
damage and oxidative stress markers, respectively. After validation in this paper,
this dedicated protocol can be used in different age categories to detect potential
cellular and subcellular effects following dental CBCT imaging.
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3.8 References
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CT in paediatric dentistry: DIMITRA project position statement. Pediatr Radiol. 2017. 18. Marcu M, Hedesiu M, Salmon B, Pauwels R, Stratis A, Oenning ACC, et al. Estimation of the radiation dose for pediatric CBCT indications: a prospective study on ProMax3D. Int J Paediatr Dent. 2018. 19. Signorelli L, Patcas R, Peltomaki T, Schatzle M. Radiation dose of cone-beam computed tomography compared to conventional radiographs in orthodontics. Journal of orofacial orthopedics = Fortschritte der Kieferorthopadie : Organ/official journal Deutsche Gesellschaft fur Kieferorthopadie. 2016;77(1):9-15. 20. Li G. Patient radiation dose and protection from cone-beam computed tomography. Imaging Sci Dent. 2013;43(2):63-9.
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21. Loubele M, Bogaerts R, Van Dijck E, Pauwels R, Vanheusden S, Suetens P, et al. Comparison between effective radiation dose of CBCT and MSCT scanners for dentomaxillofacial applications. European journal of radiology. 2009;71(3):461-8. 22. Centre for Radiation CaEH. Guidance on the safe use of dental cone bean CT (computed tomography) equipment. Oxfordshire: Health Protection Agency; 2010. 23. Theodorakou C, Walker A, Horner K, Pauwels R, Bogaerts R, Jacobs R. Estimation of paediatric organ and effective doses from dental cone beam CT using anthropomorphic phantoms. Br J Radiol. 2012;85(1010):153-60. 24. Department of Public Health EaSDoHP-F, Women and Children’s Health Cluster (FWC). Communicating radiation risks in paediatric imaging - Information to support healthcare discussions about benefit and risk. Switserland: World Health Organization; 2016. 25. Brenner DJ, Hall EJ. Computed tomography--an increasing source of radiation
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45. Kinner A, Wu W, Staudt C, Iliakis G. Gamma-H2AX in recognition and signaling of DNA double-strand breaks in the context of chromatin. Nucleic Acids Res. 2008;36(17):5678-94. 46. Riches LC, Lynch AM, Gooderham NJ. Early events in the mammalian response to DNA double-strand breaks. Mutagenesis. 2008;23(5):331-9. 47. Ciccia A, Elledge SJ. The DNA damage response: making it safe to play with knives. Mol Cell. 2010;40(2):179-204. 48. Torres-Bugarin O, Zavala-Cerna MG, Nava A, Flores-Garcia A, Ramos-Ibarra ML. Potential uses, limitations, and basic procedures of micronuclei and nuclear abnormalities in buccal cells. Dis Markers. 2014;2014:956835. 49. Spivack SD, Hurteau GJ, Jain R, Kumar SV, Aldous KM, Gierthy JF, et al. Gene-environment interaction signatures by quantitative mRNA profiling in exfoliated buccal mucosal cells. Cancer Res. 2004;64(18):6805-13.
50. Thomas P, Holland N, Bolognesi C, Kirsch-Volders M, Bonassi S, Zeiger E, et al. Buccal micronucleus cytome assay. Nat Protoc. 2009;4(6):825-37. 51. Siddiqui MS, Francois M, Fenech MF, Leifert WR. gammaH2AX responses in human buccal cells exposed to ionizing radiation. Cytometry A. 2015;87(4):296-308. 52. Sarto F, Tomanin R, Giacomelli L, Iannini G, Cupiraggi AR. The micronucleus assay in human exfoliated cells of the nose and mouth: application to occupational exposures to chromic acid and ethylene oxide. Mutat Res. 1990;244(4):345-51. 53. Lee JM, Garon E, Wong DT. Salivary diagnostics. Orthod Craniofac Res. 2009;12(3):206-11. 54. Mandel ID. Salivary diagnosis: more than a lick and a promise. Journal of the American Dental Association (1939). 1993;124(1):85-7. 55. Miller SM. Saliva testing--a nontraditional diagnostic tool. Clin Lab Sci. 1994;7(1):39-44. 56. Dame ZT, Aziat F, Mandal R, Krishnamurthy R, Bouatra S, Borzouie S, et al. The human saliva metabolome. Metabolomics. 2015;11(6):1864-83. 57. ICRP. Recommendations of the ICRP. ICRP Publication 26. 1977(Ann. ICRP 1 (3)). 58. ICRP. 1990 Recommendations of the International Commission on Radiological Protection. ICRP Publication 60. 1991(Ann. ICRP 21 (1-3). 59. ICRP. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. 2007(Ann. ICRP 37 (2-4)). 60. Stratis A. Customized Monte Carlo Modelling for Paediatric Patient Dosimetry in Dental and Maxillofacial Cone Beam Computed Tomography Imaging [Doctoral Thesis]. Leuven University Press: KU Leuven; 2018. 61. Virag P, Hedesiu M, Soritau O, Perde-Schrepler M, Brie I, Pall E, et al. Low-dose radiations derived from cone-beam CT induce transient DNA damage and persistent inflammatory reactions in stem cells from deciduous teeth. Dentomaxillofac Radiol. 2018:20170462. 62. Suetens A, Konings K, Moreels M, Quintens R, Verslegers M, Soors E, et al. Higher Initial DNA Damage and Persistent Cell Cycle Arrest after Carbon Ion Irradiation Compared to X-irradiation in Prostate and Colon Cancer Cells. Front Oncol. 2016;6:87. 63. Ghardi M, Moreels M, Chatelain B, Chatelain C, Baatout S. Radiation-induced double strand breaks and subsequent apoptotic DNA fragmentation in human peripheral blood mononuclear cells. Int J Mol Med. 2012;29(5):769-80. 64. Baselet B, Belmans N, Coninx E, Lowe D, Janssen A, Michaux A, et al. Functional Gene Analysis Reveals Cell Cycle Changes and Inflammation in Endothelial Cells Irradiated with a Single X-ray Dose. Front Pharmacol. 2017;8:213. 65. Schindelin J, Arganda-Carreras I, Frise E, Kaynig V, Longair M, Pietzsch T, et al. Fiji: an open-source platform for biological-image analysis. Nat Methods. 2012;9(7):676-82. 66. De Vos WH, Van Neste L, Dieriks B, Joss GH, Van Oostveldt P. High content image cytometry in the context of subnuclear organization. Cytometry A. 2010;77(1):64-75. 67. Munro CL, Grap MJ, Jablonski R, Boyle A. Oral health measurement in nursing research: state of the science. Biol Res Nurs. 2006;8(1):35-42. 68. Shakeri Manesh S, Sangsuwan T, Pour Khavari A, Fotouhi A, Emami SN, Haghdoost S. MTH1, an 8-oxo-2'-deoxyguanosine triphosphatase, and MYH, a DNA glycosylase,
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cooperate to inhibit mutations induced by chronic exposure to oxidative stress of ionising radiation. Mutagenesis. 2017;32(3):389-96. 69. Haghdoost S, Czene S, Naslund I, Skog S, Harms-Ringdahl M. Extracellular 8-oxo-dG as a sensitive parameter for oxidative stress in vivo and in vitro. Free Radic Res. 2005;39(2):153-62. 70. Vandevoorde C, Gomolka M, Roessler U, Samaga D, Lindholm C, Fernet M, et al. EPI-CT: in vitro assessment of the applicability of the gamma-H2AX-foci assay as cellular biomarker for exposure in a multicentre study of children in diagnostic radiology. Int J Radiat Biol. 2015;91(8):653-63. 71. El-Saghire H, Thierens H, Monsieurs P, Michaux A, Vandevoorde C, Baatout S. Gene set enrichment analysis highlights different gene expression profiles in whole blood samples X-irradiated with low and high doses. Int J Radiat Biol. 2013;89(8):628-38. 72. Sudprasert W, Navasumrit P, Ruchirawat M. Effects of low-dose gamma radiation on
DNA damage, chromosomal aberration and expression of repair genes in human blood cells. Int J Hyg Environ Health. 2006;209(6):503-11. 73. Ponzinibbio MV, Crudeli C, Peral-Garcia P, Seoane A. Low-dose radiation employed in diagnostic imaging causes genetic effects in cultured cells. Acta Radiol. 2010;51(9):1028-33. 74. Das Roy L, Giri S, Singh S, Giri A. Effects of radiation and vitamin C treatment on metronidazole genotoxicity in mice. Mutat Res. 2013;753(2):65-71. 75. Ainsbury EA, Al-Hafidh J, Bajinskis A, Barnard S, Barquinero JF, Beinke C, et al. Inter- and intra-laboratory comparison of a multibiodosimetric approach to triage in a simulated, large scale radiation emergency. Int J Radiat Biol. 2014;90(2):193-202. 76. Sangsuwan T, Haghdoost S. The nucleotide pool, a target for low-dose gamma-ray-induced oxidative stress. Radiat Res. 2008;170(6):776-83. 77. Tsuzuki T, Nakatsu Y, Nakabeppu Y. Significance of error-avoiding mechanisms for oxidative DNA damage in carcinogenesis. Cancer Sci. 2007;98(4):465-70. 78. Magnander K, Elmroth K. Biological consequences of formation and repair of complex DNA damage. Cancer letters. 2012;327(1-2):90-6. 79. Kryston TB, Georgiev AB, Pissis P, Georgakilas AG. Role of oxidative stress and DNA damage in human carcinogenesis. Mutat Res. 2011;711(1-2):193-201. 80. Gonzalez JE, Roch-Lefevre SH, Mandina T, Garcia O, Roy L. Induction of gamma-H2AX foci in human exfoliated buccal cells after in vitro exposure to ionising radiation. Int J Radiat Biol. 2010;86(9):752-9. 81. Vandevoorde C, Vral A, Vandekerckhove B, Philippe J, Thierens H. Radiation Sensitivity of Human CD34(+) Cells Versus Peripheral Blood T Lymphocytes of Newborns and Adults: DNA Repair and Mutagenic Effects. Radiat Res. 2016;185(6):580-90. 82. Deminice R, Sicchieri T, Payao PO, Jordao AA. Blood and salivary oxidative stress biomarkers following an acute session of resistance exercise in humans. Int J Sports Med. 2010;31(9):599-603. 83. da Fonte JBM, de Andrade TM, Albuquerque RLC, de Melo MDB, Takeshita WM. Evidence of genotoxicity and cytotoxicity of X-rays in the oral mucosa epithelium of adults subjected to cone beam CT. Dentomaxillofac Rad. 2018;47(2). 84. de Geus JL, Wambier LM, Bortoluzzi MC, Loguercio AD, Kossatz S, Reis A. Does smoking habit increase the micronuclei frequency in the oral mucosa of adults compared to non-smokers? A systematic review and meta-analysis. Clin Oral Investig. 2018;22(1):81-91.
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Chapter 4:
Dental cone beam CT
examination induces oxidative
damage and antioxidant
response in children’s saliva
Belmans N, Gilles L, Vermeesen R, Virag P, Hedesiu M, Salmon B, Baatout S,
Lucas S, Jacobs R, Lambrichts I, and Moreels M (2019) Dental cone beam CT
examination induces oxidative damage and antioxidant response in children’s
saliva. In review for Nature Scientific Reports
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4.1 Abstract
Assessing the possible biological effects of exposure to low doses of ionizing
radiation (IR) is one of the prime challenges in radiation protection, especially in
medical imaging. Today, radiobiological data on cone beam CT (CBCT) related
biological effects are scarce.
In children and adults, the induction of DNA double strand breaks (DSBs)
in buccal mucosa cells and 8-oxo-7,8-dihydro-2’-deoxyguanosine (8-oxo-dG) and
antioxidant capacity in saliva samples after CBCT examination were examined.
No DNA DSBs induction was observed in children nor adults. In children
only, an increase in 8-oxo-dG levels were observed 30 minutes after CBCT. At the
same time an increase in antioxidant capacity was observed in children, whereas
a decrease was observed in adults.
Our data indicate that children and adults react differently to IR doses
associated with CBCT. Fully understanding these differences could lead to an
optimal use of CBCT in different age categories as well as improved radiation
protection guidelines.
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4.2 Uncertainties concerning low dose ionizing
radiation exposure and medical imaging
Nowadays, one of the prime challenges in radiation protection is assessing
the possible biological effects of exposure to low doses of ionizing radiation (IR).
Currently, the linear non-threshold (LNT) model is used to estimate risks involved
in the low dose range. It assumes that there is no threshold dose below which no
biological effects will occur and that the risk increases linearly with the absorbed
dose.(1) Recently the LNT model has been heavily debated.(2) Although the LNT
model is supported by epidemiological evidence in the high dose range (> 100
milliGray (mGy)), increasing evidence disproves it in the low dose range.(3-5) In
addition, a lot of uncertainties still exist about low doses (< 100 mGy), due to a
lack of statistical power of the epidemiological data. This is of importance in
medical imaging applications of IR, such as computed tomography (CT) and, more
recently, cone beam computed tomography (CBCT), which typically use doses far
below 100 mGy, (typically between 0.01 – 0.10 mGy).(6-9)
Multiple controversial studies indicate that exposure of children to
diagnostic radiology may lead to radiation-induced malignancies later in life.
Retrospective studies observed that the use of CT scans in children could triple
the risk of leukaemia and brain cancers.(10-12) A 24% increase in cancer incidence
was seen in an Australian linker study, which indicated that the cancer incidence
was greater after exposure at younger ages.(13) The EPI-CT study was set up to
gain more insight into the potential adverse effects associated with CT
examinations in children.(14) Finally, it was estimated that the probability to
develop radiation-induced malignancies after CBCT exposure is 6 cases per
1,000,000 CBCT scans on average, with age at exposure and gender mostly
influencing the risk.(15, 16) Despite these potential links between diagnostic
radiology and radiation-induced malignancies, absolute evidence from prospective
studies is scarce.(3, 6) Yeh et al. (2018) estimated the risks of dental CBCT and
found that the risk of exposure-induced death (REID) values were highest in 10-
year old subjects. These REID values were two times higher than in 30-year old
subjects. The risk was higher in females than in males and the risk decreased with
increasing age.(17) Radiobiological research can help explain the uncertainties of
epidemiological studies as well as give more insights into the underlying
mechanisms.(1, 18)
Since the introduction of CBCT in the late 1990s, its use has become
widespread and is applied in several specialties in dental medicine including oral
and maxillofacial surgery, orthodontics, periodontics and dental implants.(19-21)
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Given that children are more radiosensitive than adults, this raised questions
about potential radiation-induced health effects associated with diagnostic
radiology in children.(7, 8, 22-25) IR doses associated with paediatric dental CBCT
became a major concern for the general public when the New York Times
published two articles about the topic (2010 and 2012).(26, 27) Especially in
pedodontic and orthodontics, most CBCT examinations are performed on children
(< 18 years old).(7, 25)
Exposure to IR, such as X-rays, could result in damage to important
biomolecules either directly or indirectly. The former results in direct damage (e.g.
ionization) to biomolecules. The latter leads to the generation of free radicals,
usually through hydrolysis of water. These radicals (e.g. reactive oxygen species
(ROS)) can in turn damage biomolecules in nano- to microseconds.(28)
IR can cause several types of DNA lesions, including single strand breaks,
double strand breaks (DSBs) and base alterations.(29, 30) DNA DSBs are considered
the most harmful because they are less likely to be repaired correctly.(31)
Inaccurate repair of DSBs could result in mutations, chromosome
rearrangements, chromosome aberrations and loss of genetic information.(32, 33)
Therefore, eukaryotes have developed the DNA damage response (DDR).(34) The
DDR consists of a signalling cascade that results in the recruitment of multiple
DDR proteins to the vicinity of DSBs, including histone H2AX phosphorylated on
serine 139 (γH2AX) and p53-binding protein 1 (53BP1). Both γH2AX and 53BP1
form DNA damage foci and show a quantitative relationship between the number
of foci and the number of DSBs.(35, 36)
Since more than 60% of a cell consists of water, most of the DNA damage
caused by X-rays is indirect via free radicals such as ROS (e.g. the hydroxyl
radical, superoxide radicals and hydrogen peroxide).(29, 37) An excess of ROS
causes oxidative stress in the cell which is countered by antioxidant defence
mechanisms. In the context of oral pathology, oxidative stress is associated with
periodontitis, dental caries and oral cancers.(38, 39) ROS can cause oxidative DNA
damage through oxidative base lesions, of which over 20 have been identified.(40)
An example of oxidative damage to DNA/nucleotides is 8-oxo-7,8-dihydro-2’-
deoxyguanosine (8-oxo-dG), a mutagenic base modification.(41)
The buccal mucosa (BM), which lines the oral cavity, is an easily accessible
source for collecting buccal mucosal cells (BMCs) in a minimally invasive, pain-
free way.(42) BMCs have been used to study (amongst others) the impact of
nutrition, lifestyle factors and exposure to genotoxins, including exposure to IR.(43,
44) IR-induced genotoxicity can be monitored in BMCs by measuring γH2AX levels
and can be used to monitor radiation exposure and DNA damage in radiotherapy
patients.(45, 46)
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Saliva is a bodily fluid that is secreted into the oral cavity. It originates
mainly from the parotid, submandibular and sublingual glands and is an aqueous
solution (> 99% water) containing both organic and inorganic molecules.(47)
Saliva, commonly referred to as ‘mirror of the body’, has several advantages over
other biological samples, such as blood. It is readily available, collection can be
done in a non-invasive way, and its use is very cost-effective.(48, 49) These
advantages make saliva an ideal sample to collect from paediatric patients and
for use in diagnostics.(49, 50) Currently, salivary diagnostics is becoming
increasingly important in radiation biomarker research.(48, 51) Since X-rays induce
most damage to biomolecules via ROS, measuring ROS and their effects in saliva
samples could be a feasible indicator of radiation exposure.
The main aim of our study is to characterize the short-term radiation-
induced effects associated with CBCT examinations, specifically in children. To this
end, the sub-objectives were 1) to evaluate the induction of DNA DSBs in BMCs,
and 2) to evaluate oxidative stress (by measuring 8-oxo-dG levels) as well as total
antioxidant capacity in saliva samples.(52) These were monitored in children and
adults, to identify potential age-related differences.
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4.3 Materials & Methods
4.3.1 EU OPERRA - DIMITRA study
The DIMITRA study is an non-interventional, prospective study that
focusses on radiation-induced effects related to diagnostic CBCT exposure in
children. It is a multicentre study carried out in three European centres: the Oral
and MaxilloFacial Surgery – Imaging & Pathology department (Katholieke
Universiteit Leuven, Leuven, Belgium), the Dental Medicine Department of the
Bretonneau Hospital (Paris, France) and the Iuliu Hatieganu University of Medicine
and Pharmacy (Cluj-Napoca, Romania).(52) Ethical approval was obtained at the
participating sites (B322201525196, Belgium; N°15-021, France;
208/21.04.2015, Romania).
4.3.2 Patient selection
Patients with various indications were referred to the clinic for CBCT
examination. They were examined using CBCT device settings that match their
individual needs. Thus the FOV, kV, mAs and resolution mode are adjusted to fit
with each individual’s indication and age, in agreement with the ALADAIP principle,
as described in the DIMITRA position statement by Oenning et al..(7) Throughout
the three participating centres, three CBCT devices were used: Accuitomo 170
(Mortia, Osaka, Japan), NewTom VGi evo (Cefla S.C., Imola, Italy) and Promax
3D (Planmeca OY, Helsinki, Finland).
Eligible patients were children/adolescents from 3 to 18 years old, as well
as adults (> 18 years old), with good oral hygiene. Exclusion criteria were the
presence of systemic diseases, the use of antibiotics or anti-inflammatory drugs,
smoking and not giving informed consent prior to enrolment. In case of underage
children, both parents needed to consent unless one parent has explicit permission
from the other parent.(52)
4.3.3 Buccal mucosal cell collection and immunocytological staining
The collection and staining method were described in detail by Belmans et
al. (2019).(52) Briefly, synthetic swabs were used to collect BMCs just before, 30
minutes and 24 hours after CBCT examination using a protocol modified from
Thomas et al. (2009).(42) Before each swabbing the patient rinsed his/her mouth
twice with water. The swabs were put in Saccomanno’s fixative (50% ethanol and
2% polyethylene glycol in milliQ water) and stored at 4°C. Next, the BMCs were
centrifuged at 580g for 10 minutes. Then they were washed three times in buccal
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buffer (BuBu) (0.01 M Tris-HCl, 0.1 M EDTA, 0.02 M NaCl, 1% FBS, pH = 7). Next
the BMCs were passed through a 100 µm nylon filter (Falcon®, VWR Belgium,
Leuven, Belgium). Then the BMCs were washed one last time and pelleted. The
pelleted BMCs were fixed in 500 µl of 2% paraformaldehyde (PFA) (Sigma Aldrich,
St-Louis, MO, USA). Afterwards, the BMCs were washed twice with 1x phosphate-
buffered saline (PBS) (Gibco, Life Technologies, Ghent, Belgium). Then they were
spotted on coverslips by cytocentrifugation (ThermoFisher, Waltham, MA, USA).
The coverslips were placed in 4-well culture plates (Nunc, ThermoFisher, Roskilde,
Denmark) so that the BMCs were facing up.
The BMCs were washed with 1x PBS before permeabilization with 0.25%
Triton X-100 in 1x PBS. After another washing step, the BMCs were blocked with
1x pre-immunized goat serum (ThermoFisher, Waltham, MA, USA) in 1x TBST and
0.005 g/v% TSA blocking powder (PerkinElmer, FP1012, Zaventem, Belgium)
(TNB) for 1 hour at room temperature (RT). Afterwards, the BMCs were incubated
with primary mouse monoclonal anti-γH2AX antibody (Millipore 05-636, Merck,
Overijse, Belgium) (1:300 in TNB) and rabbit polyclonal anti-53BP1 antibody
(Novus Biologicals NB100-304, Abdindon, UK) (1:1000 in TNB). Incubation was
done overnight at 4°C on a rocking platform. After incubation, the BMCs were
washed in 1x PBS. Then the BMCs were incubated for 1 hour at RT with goat anti-
mouse Alexa Fluor® 488-labelled antibody (ThermoFisher, A11001, Waltham, MA,
USA) (1:300 in TNB) and goat anti-rabbit Alexa Fluor® 568-labelled antibody
(1:1000 in TNB) (ThermoFisher, A11011, Waltham, MA, USA). Afterwards the
BMCs were washed with 1x PBS and finally the coverslips were mounted with
Prolong Diamond antifade medium with 4’,6-diamidino-2-phenylindole (DAPI)
(ThermoFisher, Waltham, MA, USA).
Finally, images were acquired with a Nikon Eclipse Ti fluorescence
microscope using a 40x dry objective (Nikon, Tokyo, Japan). Images were
analysed with open source Fiji software(53), which analyses each nucleus based on
the DAPI signal and within each nucleus the signals from Alexa Fluor® 488 and -
568 represent the γH2AX and 53BP1 foci, respectively. The number of co-localized
foci per nuclei were determined using the Cellblocks toolbox.(54)
4.3.4 Saliva collection
The collection of saliva samples was described in detail by Belmans et al
(2019) (52). In summary, saliva samples were collected right before and 30
minutes after CBCT examination using the passive drool method(55), and sampling
coincided with the BMC collection. Immediately after collection, the whole saliva
was stored at -20°C until shipment. After shipment to the lab, saliva samples were
centrifuged at 10,000g at 4°C and the supernatant was stored at -80°C until
further analysis.
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4.3.5 8-oxo-dG enzyme-linked immunosorbent assay
8-oxo-dG was analysed using a 8-oxo-dG enzyme-linked immunosorbent
assay (ELISA). Prior to this assay, 500 µl of saliva was purified twice on a C18
solid phase extraction column (Varian, Lake Forest, CA, USA) as described by
Shakeri Manesh et al. (2017).(56) The 8-oxo-dG ELISA (Health Biomarkers Sweden
AB, Stockholm, Sweden) was performed as described by Haghdoost et al
(2005).(57) In short, 270 µl of sample/standard was added to 165 µl of primary
antibody and incubated for 2 hours at 37°C on a shaker. The ELISA plate was
washed with 1x PBS and 140 µl of sample/standard was loaded per well. The plate
was incubated overnight at 4°C on a shaker. Next, the plate was washed with 1x
washing solution and 140 µl of secondary antibody was added per well. After a 2
hour incubation at RT, the plate was washed with 1x washing solution. Afterwards,
140 µl of chromogenic substrate 3,3’,5,5’-tetramethylbenzidine (One-step
substrate system, Dako, Glostrup Municipality, Denmark) was added and the plate
was incubated for 15 minutes at RT. The colour reaction was stopped by adding 2
M sulphuric acid. Finally, the absorbance was measured at 450 nm (signal) and
570 nm (background) using a microplate reader (ClarioStar, BMG Labtech,
Ortenberg, Germany).
4.3.6 Total antioxidant capacity determination
The Ferric Reducing Antioxidant Power (FRAP) assay (Cell Biolabs, CA, USA)
was performed on whole saliva according to the manufacturer’s instructions.
Briefly, 100 µl of sample/standard and 100 µl reaction reagent were added per
well of a 96-well plate. Then the plate was incubated for 10 minutes at RT on a
shaker. Finally, the absorbance was measured at 560 nm using a microplate
reader (ClarioStar, BMG Labtech, Ortenberg, Germany).
4.3.7 Dose calculations – Monte Carlo simulation
A fully validated Monte Carlo (MC) framework, which was developed by the
DIMITRA group, was used for dosimetric calculations.(58, 59) This MC simulation
relies on a database of pediatric head voxel models.(60) By using this MC DIMITRA
framework, absorbed organ doses were calculated for each individual patient.
When simulating organ doses, the normalized absorbed organ dose values are
provided in µGy/mAs. In the MC DIMITRA framework, normalized absorbed organ
doses are related to the age of the patient via the following equation:
y = a x ln(x) + b
where y is the normalized absorbed organ dose (µGy/mAs), x is the age of the
patient at the time of the scan, and the constants a and b are factors that depend
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on the organ scanned, the clinical case, and the device used.(58) Simply multiplying
the normalized absorbed organ dose by the mAs used for each specific scanning
protocol results in an absorbed organ dose value. Thus the absolute organ dose
can be calculated as follows:
yi,j = [a x ln(x) + b] x mAsj
where i represents a specific organ, and j stands for a specific examination. Note
that this equation is not validated for adults, i.e. patients older than 18 years old.
Therefore, no doses were simulated for adults using this equation.
4.3.8 Statistics
Statistical analysis was performed using GraphPad 7.02 (GraphPad Inc., CA,
USA). The results of the DNA DSBs in BMCs were analysed using repeated
measures one-way analysis of variance (ANOVA). 8-oxo-dG and FRAP assay
results were analysed using two-tailed paired t-tests. To analyse differences
between age groups and differences in radiation sensitivity, two-tailed unpaired
t-tests were performed. While all tests listed above are parametric tests, non-
parametric alternatives were used if conditions were not met. P values lower than
.05 were considered as statistically significant. Results are shown as mean ±
standard error of the mean (SEM).
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4.4 Results
4.4.1 Patients and dose exposure
In total, 147 children that participated in this study were 11 ± 3 years old
(age range: 3 – 18 years old). 73 boys and 74 girls were included. Besides, 23
adults (9 men and 14 women) that participated were 43 ± 17 years old (age
range: 19 -77 years old). Three CBCT devices were used, namely Promax 3D
(Planmeca, Finland), Accuitomo 170 (Morita, Osaka, Japan), NewTom VGi-evo
(Cefla S.C., Imola, Italy), with average (simulated) absorbed doses to the salivary
glands of 1613 ± 19 µGy, 2416 ± 324 µGy and 4283 ± 353 µGy, respectively.(61,
62) The study was approved by the ethical committees of the participating hospitals
(see Material & Methods section). All patients (or their parents, in case of children)
gave written informed consent (see supplementary data 1 and 5 and
supplementary table 1).
4.4.2 DNA double strand break detection in exfoliated buccal mucosal
cells before and after CBCT examination
The results from co-localized γH2AX and 53BP1 foci, which are a measure
for DNA DSBs, show no changes in the amount of DSBs after CBCT examination,
neither in children nor adults (figure 4.1).
In children (N = 38, degrees of freedom (DF) = 2, Friedman statistic = 2.7,
p =.2538) a slight increase was seen in the amount of foci from 0.25 ± 0.054
foci/cell before CBCT to 0.47 ± 0.12 foci/cell 30 minutes after CBCT (p > .9999).
24 hours after CBCT the amount of foci returned to baseline levels (0.3 ± 0.09
foci/cell) (p > .9999). The decrease between 30 minutes after CBCT and 24 hours
after, however, is not significant (p = .5614).
Similarly, no significant changes in the amount of co-localized γH2AX and
53BP1 foci were found in adult patients (N = 13, DF = 2, Friedman statistic = 1.0,
p = .6065). Before CBCT, 0.0014 ± 0.0014 foci/cell were counted, which increased
slightly to 0.0053 ± 0.0035 foci/cell 30 minutes after CBCT exposure (p > .9999).
Contrary to the children, the number of foci per cell remained increased 24 hours
after CBCT when compared to before CBCT (0.0061 ± 0.0051 foci/cell; p > .9999).
Between 30 minutes after CBCT and 24 hours after CBCT no significant difference
was observed (p > .9999).
Interestingly, the amount of foci per cell was significantly higher in children
than in adults at every time point. Before CBCT 0.25 ± 0.054 foci/cell were
observed in children and 0.0014 ± 0.0014 foci/cell were observed in adults (Mann-
Whitney U value = 121, p = .0020). 30 minutes after CBCT, the amount of foci in
children (0.47 ± 0.12 foci/cell) was significantly higher than the amount seen in
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adults (0.0053 ± 0.0035 foci/cell) (Mann-Whitney U value = 145, p = .0146).
Finally, 24 hours after CBCT exposure the amount of foci in children (0.3 ± 0.09
foci/cell) was higher than the amount of foci in adults (0.0061 ± 0.0051 foci/cell)
(Mann-Whitney U value = 170, p = .0487).
Since both children and adults showed an increase 30 minutes after CBCT,
these increases were compared (# foci/cell30 minutes after CBCT - # foci/cellbefore CBCT).
The mean increase in children (0.17 ± 0.097 foci/cell) did not differ from the
increase in adults (0.0078 ± 0.01 foci/cell) (Mann-Whitney U value = 412, p =
.8089). Regarding the difference between 30 minutes after CBCT and 24 hours
after, no significant difference was observed between children (-0.17 ± 0.11
foci/cell) and adults (0.00087 ± 0.0066 foci/cell) (Mann-Whitney U value = 196,
p = .2105).
Figure 4.1. No DNA double strand breaks (DSBs) are induced in buccal mucosal
cells (BMCs) after cone beam computed tomography (CBCT) examination, neither
in children nor in adults. No significant increases in the amount of γH2AX/53BP1 co-
localized foci were observed 30 minutes and 24 hours after CBCT examination in children
(Black dots; N = 38, degrees of freedom = 2, Friedman statistic = 2.7, p = .2538) and in
adults (Red dots; N = 13, degrees of freedom = 2, Friedman statistic = 1.0, p = .6065).
Before (Mann-Whitney U value = 121, p = .0020), 30 minutes after (Mann-Whitney U value
= 145, p = .0146) and 24 hours after CBCT (Mann-Whitney U value = 170, p = .0487) the
amount of DSBs was significantly higher in children then in adults. Only the data from
patients of which results were obtained for all time points were included. Green dotted line
= average number of foci; * = p ≤ .05; ** = p ≤ .0021.
4.4.3 8-oxo-dG levels in saliva samples
8-oxo-dG levels were measured in saliva samples collected before and after
CBCT examination. They were increased in children but not in adults 30 minutes
after CBCT (figure 4.2).
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In children, a significant increase in 8-oxo-dG levels was observed between
samples taken before CBCT examination (1.86 ± 0.26 ng/ml) and 30 minutes
after CBCT (4.11 ± 0.62 ng/ml) (N = 68, DF = 67, t value = 4, p < .0001), an
average increase of 121 % (figure 4.2; supplementary data 3). In adults , an
increase from 1.52 ± 0.34 ng/ml 8-oxo-dG before CBCT to 2.42 ± 0.55 ng/ml 30
minutes after CBCT was observed (N = 19, DF = 18, t value = 1.58, p = .1317),
resulting in an average increase of 59% (figure 4.2). No differences were observed
between the values of children and adults before CBCT (Mann-Whitney U value =
643.5, p = .98) and 30 minutes after CBCT (Mann-Whitney U value = 622.5, p =
.81).
In the group of children, data were split based on gender (Table 4.1). Both
in boys and girls the amount of 8-oxo-dG increased significantly after CBCT
examination (N = 35, p = .024 and N = 33, t-value = 2.91, DF = 32, p =.0065,
respectively). Furthermore, no differences between boys and girls was observed
(Table 4.1). This was confirmed when the proportional change between values
before and after CBCT were compared between boys and girls (p = .6907) (see
supplementary data 2).
Figure 4.2. Excretion of 8-oxo-7,8-dihydro-2’-deoxyguanosine (8-oxo-dG) into saliva is increased after cone beam computed tomography (CBCT) examination in children but not in adults. Only data from patients of which results were obtained for both time points were included. In children there is a significant average increase of 121% in 8-oxo-dG excretion 30 minutes after CBCT examination (N = 68, DF = 67, t value = 4, p < .0001). In adults there is an average increase in 8-oxo-dG excretion of 59% (N = 19, DF = 18, t value = 1.58, p = .1317). Green dotted line = average; ****= p < .0001.
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Table 4.1. Comparison between boys and girls for 8-oxo-dG excretion before and
after cone beam computed tomography (CBCT) examination.
Boys (N = 35)
Girls (N = 33)
P value t-value Degrees of
freedom
8-oxo-dG (ng/ml)
Before CBCT 1.71 ± .27 2.01 ± .46 .63
Mann-Whitney U value = 537.5
N.A.
8-oxo-dG (ng/ml)
30 minutes after CBCT
4.21 ± .94 4.01 ± .83 .96
Mann-Whitney U value = 573.5
N.A.
P value .024
.0065
t-value (Wilcoxon
test) 2.9
Degrees of freedom
(Wilcoxon test)
32
Plotting the proportional change in 8-oxo-dG levels of children against the
absorbed dose received by the patients showed no visible trend or dose response
(figure 4.3).
Figure 4.3. No dose response in 8-oxo-dG excretion in saliva 30 minutes after cone beam computed tomography in children. No visible dose response (linear or otherwise) was observed in 8-oxo-dG excretion in children. Radiation doses were the absorbed doses at
the salivary glands as calculated by MC simulations.(60,
61)
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4.4.4 Total antioxidant capacity in saliva samples
Ferric Reducing Antioxidant Power (FRAP) values were measured in saliva
samples before and 30 minutes after CBCT examination. They were significantly
increased in children and decreased significantly in adults 30 minutes after CBCT
examination (figure 4.4).
Children showed a slight, but significant increase in FRAP value after CBCT
examination. Thirty minutes after CBCT examination, FRAP values increased from
260.80 ± 11.87 to 277.90 ± 13.22, an increase of about 7% (N = 117, t-value =
1.98, DF = 116, p = .0498) (supplementary data 4). Contrary to the results in
children, a decrease of about 9% in FRAP values was found in adults. FRAP values
decreased from 202.90 ± 21.28 at baseline to 185.50 ± 20.74 30 minutes after
CBCT examination (N = 17, t-value = 2.22, DF = 16, p= .0412). No significant
differences were observed between children and adults before CBCT examination
(t-value = 1.80, DF = 132, p = .0747). However, the FRAP values 30 minutes
after CBCT examination were significantly higher in children than in adults (Welch-
corrected t-value = 3.76, DF = 30.93, p= .0007). The response in children and
adults differed significantly when comparing the average increase in children with
the average decrease in adults (Welch-corrected t-value = 2.96, DF = 65, p =
.0043).
Figure 4.4. Ferric reducing antioxidant power (FRAP) values increase in saliva samples from children after cone beam computed tomography (CBCT) examination, while decreasing in saliva samples from adults. In children (black violin plots) a significant increase in FRAP values was observed 30 minutes after CBCT examination (N = 117, t-value = 1.98, degrees of freedom (DF) = 116, p = .0498). In adults (red violin plots) a significant decrease was observed 30 minutes after CBCT examination (N = 17, t-value = 2.22, DF = 16, p= .0412). The FRAP values 30 minutes after CBCT are significantly higher in children than in adults (Welch-corrected t-value = 3.76, DF = 30.93, p= .0007). The response in children and adults differs significantly, with an average increase of 17.10 ± 8.62 in children and an average decrease of 17.40 ± 7.84 in adults (Welch-corrected t-value = 2.96, DF = 65, p = .0043). * = p ≤ .05; *** = p ≤ .0002.
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Table 4.2. Comparison between boys and girls FRAP values before and after cone
beam computed tomography (CBCT) examination.
Boys (N = 62)
Girls (N = 55)
P value t-value Degrees of
freedom
FRAP value Before CBCT
265.90 ± 19.39
263.00 ± 16.85
.9318 0.086 132
FRAP value 30 minutes after CBCT
277.00 ± 22.84
295.40 ± 18.35
.4963 0.68 132
P value .4194
.0268
t-value 0.81 2.28 Degrees of Freedom
61 54
Results were also analysed based on gender (table 4.2). In children, both
boys and girls showed an increase in FRAP values, but the increase was only
significant in girls (N = 62, t-value = 0.81, DF = 61, p = .4194 and N = 55, t-
value = 2.28, DF = 54, p = .0268, respectively). Additionally, in both adult men
and women a decrease was observed, but this was also only significant for women
(N = 4, Wilcoxon test, p > .9999 and N = 13, t-value = 2.27, DF = 12, p = .0428,
respectively). Furthermore, in children it was observed that the baseline levels
were lower in the morning (225.10 ± 12.48) than baseline levels in the afternoon
(282.30 ± 21.04) (Welch-corrected t-value = 2.34, DF = 82.42, p = .0217). The
same was observed in adults (baseline morning: 174 ± 21; baseline afternoon:
269 ± 42), although this difference was not statistically significant (Mann-Whitney
U value = 12, p = .0897). Therefore, the data from children were split into a
morning and afternoon group. The salivary FRAP values did not significantly differ
after CBCT examination if data were corrected for time of sample collection. In
the morning groups, there was no significant change in both boys and girls (N =
24, Wilcoxon test, p = .97 and N = 10, t-value = 0.81, DF = 9, p = .7394,
respectively). In the afternoon group, FRAP levels in boys did not change (N = 17,
Wilcoxon test, p = .89). However, in girls from the afternoon group FRAP levels
increased significantly (N = 24, t-value = 2.14, DF = 23, p = .0431).
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4.5 Discussion
Determining the biological effects of exposure to low doses of IR, such as
those used in medical imaging, of paramount concern in radiation protection
today. This study aimed to characterize the short-term radiation-induced effects
associated with CBCT examinations, specifically in children. To this end, the
number of DNA DSBs was monitored in BMCs and 8-oxo-dG levels as well as total
antioxidant capacity were monitored in saliva samples using previously optimized
protocols.(52) We report that no induction of DNA DSBs was detected in BMCs,
neither in children nor in adults. Furthermore, a significant increase in 8-oxo-dG
and total antioxidant capacity was observed in saliva samples from children 30
minutes after CBCT examination. In contrast, no significant changes were
observed in 8-oxo-dG levels in adults. Furthermore, a significant decrease in total
antioxidant capacity was observed in saliva samples from adults 30 minutes after
CBCT examination. Since no dose response was observed, the outcome of this
study could help to clarify the controversy surrounding the LNT model as well as
the uncertainty about potential adverse health effects after exposure to low doses
of IR (< 100 mGy), such as those used in CBCT. Finally, the data from DNA DSBs
after 24 hours also indicate that no delayed increase in the number of DSBs occurs
after CBCT examination.
Exposure to IR can result in DSBs, which are considered very harmful, since
inaccurate repair could result in mutations, chromosome rearrangements,
chromosome aberrations and loss of genetic information.(29, 30, 32, 33) Our results
indicate that exposure to radiation doses used in CBCT examinations (0.184 mGy
–9.008 mGy in this study) does not induce DNA DSBs in BMCs from children and
adults, as observed using a microscopic γH2AX/53BP1 co-localization assay. This
assay was performed on samples collected before, 30 minutes and 24 hours after
CBCT examination. Previously, both the γH2AX assay and the γH2AX/53BP1 assay
were used to detect DNA DSBs after exposure to radiation doses used in diagnostic
and interventional radiology, such as CT scans.(63-65) These studies report a
significant increase in γH2AX foci in lymphocytes 1 hour after CT examination,
which uses higher radiation doses than CBCT. Furthermore, our group recently
showed that low doses associated with CBCT examinations are capable of inducing
DNA DSBs in vitro in dental stem cells.(66) BMCs have also been used successfully
as a biomarker for genotoxic effects, including using the γH2AX assay to detect
radiation-induced DNA DSBs.(45, 67, 68) These studies report increase of genotoxic
effects in BMCs after low dose IR exposure. Gonzalez et al. (2010) showed that in
vitro exposure of BMCs to IR induces γH2AX foci.(45) Our findings indicate that
CBCT examinations do not cause DNA DSBs in BMCs, which is in line with previous
publications focusing on genotoxicity induced by radiological examinations. In
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these studies, no genotoxic effects, i.e. micronucleated cells, were observed after
low doses of IR, such as panoramic dental radiology and CBCT. These studies,
however, all reported increases in other nuclear alterations (e.g. pyknosis,
karyorrhexis and karyolysis) that are associated with increased cytotoxicity.(68-71)
Recently, Preethi et al. (2016) reported significant increases in the number of
micronucleated cells in BMCs after dental radiography in paediatric patients.(67)
Furthermore, Yoon et al. (2009) reported a significant increase in γH2AX foci in
BMCs of adults after dental radiography.(72)
Our data show 0.0014 ± 0.0014 co-localized γH2AX/53BP1 foci per cell in
BMCs from adults at baseline. This number is remarkably lower than the 0.08 ±
0.02 γH2AX foci per cell in non-irradiated BMCs reported previously by Gonzalez
et al. (2010) (45). These different observations can be explained by the higher
sensitivity of the γH2AX/53BP1 co-staining, which eliminates the detection of
γH2AX foci observed during S-phase replication fork stalling (73). In addition,
Gonzalez et al. (2010) treated the BMCs differently, e.g. after collection they
incubated the BMCs in cell growth medium at 37° Celsius, which can also affect
the number of foci counted.(45)
Interestingly, we found before CBCT examination, but also 30 minutes and
24 hours after CBCT examination, the average number of γH2AX/53BP1 foci per
cell was higher in children than in adults. This observation contradicts what has
been published before, namely that aging is associated with accumulation of DNA
damage.(74, 75) One would expect the level of DNA damage, at least before CBCT
examination, to be higher in adults than in children. However, BMCs are the first
barrier in the inhalation and ingestion routes. Therefore, they are exposed to
several genotoxins. These can be found in environmental and lifestyle factors such
as diet, mouthwash, smoke, air pollution, etc..(76-78) These factors can, at least
partially, explain our observation, since children are more sensitive to these type
of genotoxins compared to adults due to age-related differences in absorption,
metabolism, development and body functions.(77)
Finally, we observed that the response after CBCT examination in children
did not differ significantly from that of adults. This indicates that BMCs from
children after CBCT examination do not show an increased radiosensitivity
compared to BMCs from adults.(22-24) These findings are in line with results from
Ribeiro et al. (2008). They compared the genotoxic and cytotoxic effects of dental
radiography between children and adults and found no significant differences in
micronucleus frequency or cytotoxicity.(79) However, the radiation doses used in
radiography are lower than those used in CBCT, thus this should be interpreted
with caution.
The mutagenic base modification 8-oxo-dG is a marker for oxidative
damage to DNA/nucleotides and a useful biomarker of exposure to high doses of
IR.(41, 57) This study shows that 8-oxo-dG levels excreted in saliva increased in
children but not in adults 30 minutes after CBCT. Oxidative stress has been linked
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to oral diseases such as periodontitis, dental caries and oral cancers.(38, 39)
Because of its mutagenic potential, excretion of 8-oxo-dG depends on cellular DNA
repair mechanisms, such as nucleotide excision repair, nucleotide incision repair
and Nudix hydrolase activity.(80) Therefore, a reduced DNA repair capacity may
result in accumulation of 8-oxo-dG in the cells, thus resulting in a decrease in 8-
oxo-dG excretion. Since DNA repair capacity was shown to decrease with age, this
could explain why the concentration of 8-oxo-dG in saliva samples of adults was
not increased significantly after CBCT examination, as it was in children.(81, 82)
Despite the significant increase in children and the limited increase in adults, no
statistical differences were observed between both groups. This is most likely due
to the limited group size of the adult group.
Previously, an association between the excretion of 8-oxo-dG and high
radiation doses was described.(57) This association was not linear and showed
saturation between 0.5 and 1 Gy. However, such dependency was not observed
in this study, for example children that were exposed to 0.8 mGy showed a similar
increase in 8-oxo-dG excretion as children exposed to 0.2 mGy. These data
indicate that there is a high variability in individual radiosensitivity in our study
population. Alternatively, it could be that the very low IR doses associated with
CBCT elicit a small biological response which is unrelated to the IR dose, like an
all-or-nothing mechanism. This is similar to the use of a ‘priming dose’ in adaptive
response studies. Here a very low dose of a stressor (e.g. a chemical or IR) results
in a small response which in turn prepares cells to an exposure of the same
stressor at a higher dose.(83) Our results mimic the effects seen when applying
such a ‘priming dose’.
Although 8-oxo-dG was proposed as a marker for radiosensitivity, evidence
is lacking or comes from radiotherapy patients, who receive doses that are a lot
higher than the doses in our study population.(84)
We describe for the first time that salivary 8-oxo-dG levels are significantly
increased in both boys and girls after CBCT examination. No significant gender
differences in salivary 8-oxo-dG levels were observed. Previous measurements in
urine and other cells showed similar results.(85-87) To the best of our knowledge,
similar findings of 8-oxo-dG secretion in saliva in children were not reported
before. Previous studies analysed oxidative stress markers in adults. These
studies reported higher ROS production and oxidative stress biomarkers in men
when compared to premenopausal women (reviewed by Kander et al. (2017)(88)).
It is noteworthy that these studies are all related to cardiovascular diseases and
not radiation exposure. However, there are studies that report higher oxidative
status in females which contradicts the aforementioned studies.(89).
FRAP values give information about the total antioxidant capacity of
biological samples. Our data shows on opposite response between children and
adults 30 minutes after CBCT examination: salivary FRAP values increase
significantly in children, whilst they decrease significantly in adults. Furthermore,
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the response in children is significantly different from that in adults, indicating
that children react differently to CBCT-associated radiation exposure.
Interpretation of the data needs to be done cautiously, since the data show that
the time of sampling (in the morning or in the afternoon) significantly affected the
baseline salivary FRAP values in children. The highest values were measured in
the afternoon. Similar circadian changes in FRAP values were observed before.(90)
After correcting for time of sampling, no significant changes in salivary FRAP levels
were observed, except for girls that were sampled in the afternoon. However,
since pair-wise tests were used, this circadian influence is expected to be limited
in this study.
Total antioxidant capacity has been used previously as a salivary biomarker
related to periodontal disease and dental caries. Decreases in total antioxidant
capacity have been linked to periodontal disease.(91)
The use of total antioxidant capacity as a biomarker has several limitations.
Firstly, the total antioxidant capacity that is measured is the result of a complex
mixture of antioxidants that is present in saliva. The major antioxidant in saliva
has been reported to be uric acid, which accounts for more than 85% of the
salivary antioxidant capacity. In addition, a wide array of other potent antioxidants
are found in saliva, such as superoxide dismutase, catalase, glutathione
peroxidase, ascorbic acid, several vitamins and albumin.(92, 93) In this regard,
future analysis into the enzymatic activity of specific antioxidant enzymes, e.g.
superoxide dismutase might be interesting. Secondly, a lot of biological variability
of salivary total antioxidant capacity exists. We report an average salivary FRAP
value of 202.90 ± 21.28 in adults at baseline, whereas an average of 610.83 ±
4.52 was reported before in healthy adults.(94) It is noteworthy that this patient
population was Asian, where ours is European, which may suggest ethnical
differences in salivary FRAP values. Finally, several confounding factors have been
described that affect the saliva composition and can thus affect the total
antioxidant capacity. Confounding factors may include circadian rhythm, gender,
age and diet.(90, 92) This study also found an effect of circadian rhythm (see above),
age and gender. Girls show a significant increase in salivary FRAP values, whereas
women show a significant decrease. Both boys and men showed a change (an
increase and decrease, respectively), but this was not significant. These findings
indicate that females are more susceptible to changes in total antioxidant capacity
following IR exposure and that the net effects depends on the age of the
individual. However, it is important to note that our patient group is relatively
small (N = 72 for girls and N = 13 for women). Increasing the sample size could
therefore yield different results. These limitations could interfere with
interpretation of the results. Therefore, it is important to take these confounding
factors into account during the design of a study. As with 8-oxo-dG, no dose
response relationship was observed for FRAP values.
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In conclusion, our data provide evidence that CBCT examinations cause
oxidative damage in children, as well as an increase in the antioxidant response.
In adults, a slight increase in oxidative damage and a significant decrease in the
antioxidant response were observed. These results indicate that children and
adults react differently to low doses of IR associated with CBCT examinations.
Despite this increase in oxidative damage, no induction of DNA DSBs in BMCs was
observed in children nor in adults. Furthermore, we observed some gender-related
differences. Girls/women showed a significant increase/decrease in FRAP values
after CBCT examination, whereas boys/men do not. Our data demonstrate that
saliva can be used for biomonitoring after IR exposure even if the radiation doses
are very low (< 1 mGy). However, no dose response relationship was found,
neither for 8-oxo-dG levels nor for FRAP values.
Nonetheless, these results should raise awareness about radiation
protection and the ‘As-Low-as- Diagnostically Acceptable being indication-oriented
and patient-specific’ (ALADAIP) principle among clinicians and radiologists.(7)
However, this should be investigated into more depth to gather more information
about the potential link between possible biological effects and the CBCT settings
that were used. Furthermore, the effects observed and described in this study are
short-term effects, i.e. within 30 minutes after CBCT examination. We can
conclude that adverse effects, although very small, occur and that further
research is warranted. These findings are an incentive for continuing research into
the biological effects after CBCT examination, since fully understanding them
could lead to an optimal use of CBCT in a paediatric population as well as improved
radiation protection guidelines.
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4.6 Competing interests
The authors declare that there are no competing interests.
4.7 Acknowledgements
The authors like to thank all patients (and their parents) for their willingness
to contribute to this study. They also like to express their gratitude towards the
hospital staff, especially Christelle Lefevre and the CRB facility (Dr. Sarah Tubiana,
HUPNVS – APHP, France) for their indispensable help with the sample collection.
The DIMITRA project has received funding from the European Atomic
Energy Community’s Seventh Framework Programme FP7/2007–2011 under
grant agreement no 604984 (OPERRA: Open Project for the European Radiation
Research Area).
The DIMITRA Research Group that contributed to this paper consists of N.
Belmans, M. Moreels, S. Baatout, B. Salmon, A.C. Oenning, C. Chaussain, C.
Lefevre, M. Hedesiu, P. Virag, M. Baciut, M. Marcu, O. Almasan, R. Roman, A.
Porumb, C. Dinu, H. RotaruC. Ratiu, O. Lucaciu, B. Crisan, S. Bran, G. Baciut, R.
Jacobs, H. Bosmans, R. Bogaerts, C. Politis, A. Stratis, R. Pauwels, K. de F.
Vasconcelos, L. Nicolielo, G. Zhang, E. Tijskens, M. Vranckx, A. Ockerman, E.
Claerhout, E. Embrechts.
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4.8 References
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4.8 Supplementary Data
4.8.1 Supplementary Data 1
Absorbed radiation dose does not only depend on the device uses, but also on the field of view (FOV) and scanning protocol used during the examination. (left). Patients examined using a Promax 3D device receive on average a higher radiation dose than those examined with a Accuitomo 170 device or NewTom device. However,
these data do not take into account the FOV or the scanning protocol. (right). Radiation dose increases with increasing FOV and resolution of the scan. This is seen for all devices (except for Accuitomo 170 for which only one scanning protocol was used). Furthermore, the radiation dose is higher when high resolution (HiRes) protocols were used. This explains the differences seen in the left panel, since for Planmeca ProMax and NewTom HiRes protocols were used, whereas only standard protocols were used in Accuitomo. No significances were shown in the right panel. ***: p < .0002; ****: p < .0001.
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4.8.2 Supplementary Data 2
Comparison of % change in 8-oxo-dG excretion shows no difference between boys and girls. The proportional change in 8-oxo-dG levels does not differ between boys and girls (Mann-Whitney U value = 431, p = .203). Green dotted line = average.
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4.8.3 Supplementary Data 3
8-oxo-dG (ng/ml) concentration shown per individual pediatric patient. On the x-axis, individual patients are shown, ranked from smallest change in 8-oxo-dG concentration following cone beam computed tomography (CBCT) examination to largest change in 8-oxo-dG concentration following CBCT examination (black dots). The blue dots show the 8-oxo-dG concentration prior to CBCT. The red dots show the 8-oxo-dG concentration post CBCT.
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4.8.4 Supplementary Data 4
Ferric
Red
ucin
g A
nti
oxid
an
t P
ow
er (
FR
AP
) v
alu
es
sh
ow
n p
er
ind
ivid
ual
ped
iatr
ic p
ati
en
t. O
n t
he x
-axis
, in
div
idual
patients
are
show
n,
ranked fr
om
sm
allest
change in
FRAP valu
e fo
llow
ing cone beam
com
pute
d to
mogra
phy (C
BCT)
exam
ination t
o larg
est
change in F
RAP v
alu
e f
ollow
ing C
BCT e
xam
ination (
bla
ck d
ots
). T
he b
lue d
ots
show
the F
RAP v
alu
es
prior
to C
BCT. The r
ed d
ots
show
the F
RAP v
alu
es p
ost
CBCT.
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4.8.5 Supplementary Data 5
Absorbed radiation dose to the oral mucosa plotted against the age of the patient at the time of CBCT examination. (large) Monte Carlo simulated absorbed dose to the oral mucosa is plotted against the age of the patient at the time of the CBCT examination. This graph shows that most high resolution (i.e. high mAs) (red dots) protocols are performed in children. (insert) The absorbed dose is significantly higher in children compared to adults for standard scanning protocols (green dots). Furthermore, in children, the dose used in high resolution protocols (red dots) is significantly higher than that of standard imaging protocols. Note that the Monte Carlo framework used for dose calculations is not validated for adults and thus these data might deviate from actual values. *: p < .05; ****: p < .0001; HiRes = High resolution protocol.
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4.8.6 Supplementary Table 1
Supplementary table 1. Individual patient study parameters of included patients.
Patient
ID
Age
(years) Gender*
CBCT
device**
Site of
examination
Simulated absorbed
dose(58)*** (µGy)
A1 58 M NewTom KU Leuven A2 62 F NewTom KU Leuven
A3 32 F NewTom KU Leuven A4 36 F NewTom KU Leuven
A5 62 F NewTom KU Leuven A6 26 M NewTom KU Leuven A7 44 M NewTom KU Leuven A8 22 M NewTom KU Leuven
A9 19 F NewTom KU Leuven A10 64 F NewTom KU Leuven A11 46 F NewTom KU Leuven A12 35 F Accuitomo KU Leuven A13 53 F NewTom KU Leuven A14 57 F NewTom KU Leuven A15 41 F NewTom KU Leuven
A16 30 F NewTom KU Leuven A17 30 M NewTom KU Leuven A18 71 M NewTom KU Leuven A19 27 F NewTom KU Leuven A20 24 F NewTom KU Leuven A21 34 M NewTom KU Leuven A22 Not
known M NewTom KU Leuven
A23 77 M NewTom KU Leuven C1 8 M NewTom KU Leuven 6788 C2 7 M NewTom KU Leuven 4850 C3 Not
known F NewTom KU Leuven 3565
C4 8 M NewTom KU Leuven 6182 C5 10 M NewTom KU Leuven 4610 C6 16 M NewTom KU Leuven / C7 9 M NewTom KU Leuven 2141 C8 9 F NewTom KU Leuven 1168 C9 9 F NewTom KU Leuven 1168 C10 9 F NewTom KU Leuven 6811
C11 8 M NewTom KU Leuven 11206 C12 3 M NewTom KU Leuven / C13 9 F NewTom KU Leuven 2141
C14 14 F NewTom KU Leuven 3518 C15 14 M NewTom KU Leuven 6232 C16 7 M NewTom KU Leuven 9628 C17 12 M NewTom KU Leuven 1380
C18 11 F NewTom KU Leuven 9137 C19 9 M NewTom KU Leuven 2328
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Patient ID
Age (years)
Gender* CBCT
device** Site of
examination
Simulated absorbed
dose(58)*** (µGy)
C20 11 M NewTom KU Leuven 1208 C25 13 F Accuitomo KU Leuven 1803
C28 13 M NewTom KU Leuven 1288 C30 10 M NewTom KU Leuven 1433 C32 12 F NewTom KU Leuven 1918 C33 13 F NewTom KU Leuven 1803 C34 9 F Accuitomo KU Leuven 2328
C35 8 F Accuitomo KU Leuven 2496 C36 15 F NewTom KU Leuven 4503
C37 8 F Accuitomo KU Leuven 2496 C38 12 M Accuitomo KU Leuven 1918 C39 12 F NewTom KU Leuven 4110 C40 10 M Accuitomo KU Leuven 2178 C41 10 M NewTom KU Leuven 7460 C42 9 M Accuitomo KU Leuven 2328 C43 9 M Accuitomo KU Leuven 2328
C44 8 F Accuitomo KU Leuven 2496 C45 7 F Accuitomo KU Leuven 2687 C46 9 M Accuitomo KU Leuven 2328
C47 10 F Accuitomo KU Leuven 2178 C48 10 M Accuitomo KU Leuven 2178 C49 9 M Accuitomo KU Leuven 2328
C50 10 M Accuitomo KU Leuven 2178 C51 10 M Accuitomo KU Leuven 2178 C52 10 F Accuitomo KU Leuven 2178 C53 11 M Accuitomo KU Leuven 2042 C54 10 F Accuitomo KU Leuven 2178 C55 12 M NewTom KU Leuven 999 C56 13 F Accuitomo KU Leuven 2302
C57 14 M Accuitomo KU Leuven 1698
C58 7 M NewTom KU Leuven 8773 C59 12 F NewTom KU Leuven 6826 C60 13 M NewTom KU Leuven 5211 C61 11 M Promax Bretonneau 8590 C62 9 M Promax Bretonneau 4580 C63 14 F Promax Bretonneau 3777
C64 7 M Promax Bretonneau 2014 C65 11 F Promax Bretonneau 4215 C66 10 F Promax Bretonneau 6982 C67 7 F Promax Bretonneau 3973 C68 15 F Promax Bretonneau 9460 C69 9 M Promax Bretonneau 4580
C70 6 F Promax Bretonneau 4726 C71 12 M Promax Bretonneau 9279 C72 10 F Promax Bretonneau 4388 C73 13 M Promax Bretonneau 3912 C74 7 M Promax Bretonneau 4477
Chapter 4: Dental cone beam CT examination induces oxidative damage and antioxidant response in
children’s saliva
123
Patient ID
Age (years)
Gender* CBCT
device** Site of
examination
Simulated absorbed
dose(58)*** (µGy)
C75 10 M Promax Bretonneau 2731 C76 12 M Promax Bretonneau 4057
C77 13 F Promax Bretonneau 3912 C78 15 F Promax Bretonneau 8600 C79 9 F Promax Bretonneau 4580 C80 7 M Promax Bretonneau 6492 C81 12 F Promax Bretonneau 4057
C82 13 M Promax Bretonneau 8132 C83 13 F Promax Bretonneau 3912
C84 14 M Promax Bretonneau 5805 C85 5 F Promax Bretonneau 9556 C86 8 F Promax Bretonneau 4794 C87 15 F Promax Bretonneau 4058 C88 7 M Promax Bretonneau 9828 C89 14 F Promax Bretonneau 4197 C90 14 M Promax Bretonneau 3777
C91 9 M Promax Bretonneau 4580 C92 15 F Promax Bretonneau 4058 C93 13 M Promax Bretonneau 8132
C94 10 M Promax Bretonneau 8851 C95 10 F Promax Bretonneau 4388 C96 13 F Promax Bretonneau 9036
C97 9 F Promax Bretonneau 4580 C98 9 F Promax Bretonneau 9140 C99 13 F Promax Bretonneau 9036 C100 14 F Promax Bretonneau 7929 C101 15 F Promax Bretonneau 7740 C102 10 M Promax Bretonneau 4388 C103 8 F Promax Bretonneau 9462
C104 11 F Promax Bretonneau 8590
C105 13 M Promax Bretonneau 9036 C106 14 M Promax Bretonneau 4838 C107 11 M Promax Bretonneau 8590 C108 12 M Promax Bretonneau 4057 C109 13 M Promax Bretonneau 9036 C110 10 M Promax Bretonneau 4388
C111 10 M NewTom 3G Iuliu Hatieganu +
C112 8 F NewTom 3G Iuliu Hatieganu + C113 10 M NewTom 3G Iuliu Hatieganu + C114 10 M NewTom 3G Iuliu Hatieganu + C115 11 F NewTom 3G Iuliu Hatieganu + C116 9 F NewTom 3G Iuliu Hatieganu +
C117 13 F NewTom 3G Iuliu Hatieganu + C118 15 F NewTom 3G Iuliu Hatieganu + C119 6 F NewTom 3G Iuliu Hatieganu + C120 8 F NewTom 3G Iuliu Hatieganu + C121 15 M NewTom 3G Iuliu Hatieganu +
Chapter 4: Dental cone beam CT examination induces oxidative damage and antioxidant response in
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124
Patient ID
Age (years)
Gender* CBCT
device** Site of
examination
Simulated absorbed
dose(58)*** (µGy)
C122 8 M NewTom 3G Iuliu Hatieganu + C123 18 F NewTom 3G Iuliu Hatieganu +
C124 8 M NewTom 3G Iuliu Hatieganu + C125 9 F NewTom 3G Iuliu Hatieganu + C126 14 F NewTom 3G Iuliu Hatieganu + C128 12 F Promax Iuliu Hatieganu 10655 C129 12 F Promax Iuliu Hatieganu 5011
C130 12 F Promax Iuliu Hatieganu 4748 C131 16 M Promax Iuliu Hatieganu 7334
C132 13 F NewTom 3G Iuliu Hatieganu + C133 6 M Promax Iuliu Hatieganu 6920 C134 15 F Promax Iuliu Hatieganu 4154 C135 17 F Promax Iuliu Hatieganu 5218
C136 10 Not
known Not known Iuliu Hatieganu
C137 9 M NewTom 3G Iuliu Hatieganu +
C138 8 F Promax Iuliu Hatieganu 8742 C139 10 M Promax Iuliu Hatieganu 6968 C140 10 M Promax Iuliu Hatieganu 8554
C141 12 M Promax Iuliu Hatieganu 4001 C142 13 F Promax Iuliu Hatieganu 6832 C143 13 F Promax Iuliu Hatieganu 3431
C144 13 F Promax Iuliu Hatieganu 3808 C145 5 M Promax Iuliu Hatieganu 1898 C146 12 F Promax Iuliu Hatieganu 5011 C148 10 M NewTom 3G Iuliu Hatieganu + C149 9 M Promax Iuliu Hatieganu 3290 C150 16 M Promax Iuliu Hatieganu 3363 C151 12 F Promax Iuliu Hatieganu 2251
C152 14 F Promax Iuliu Hatieganu 3661
C153 10 F Promax Iuliu Hatieganu 2814 C154 17 M Promax Iuliu Hatieganu 4050
*: F = female; M = male
**: NewTom = NewTom VGi-evo; Promax = Promax 3D; Accuitomo = Accuitomo 170
***: Absorbed dose calculated for the oral mucosa +: No dose simulations were performed for NewTom 3G
Chapter 5: in vitro assessment of the DNA damage response in dental stem cells following low dose X-
ray exposure
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Chapter 5:
In vitro assessment of the DNA
damage response in dental stem
cells following low dose X-ray
exposure
Belmans N, Gilles L, Welkenhuysen J, Vermeesen R, Salmon B, Baatout S, Jacobs
R., Lucas S, Lambrichts I, Moreels M In vitro assessment of the DNA damage
response in dental stem cells following low dose X-ray exposure. In final
preparation – To be submitted September 2019
Chapter 5: in vitro assessment of the DNA damage response in dental stem cells following low dose X-
ray exposure
127
5.1 Abstract
Mesenchymal stem cells (MSCs) are crucial for tissue homeostasis.
Therefore assuring their genomic stability is essential. Exposure of stem cells to
ionizing radiation (IR) is potentially detrimental for normal tissue homeostasis.
Although it has been established that exposure to high doses of IR has severe
adverse effects in MSCs, knowledge about the impact of low doses of IR is lacking.
However, knowing the impact of low doses of IR is important for several MSC
types, such as dental MSCs, due to the increasing use of (dental) imaging that
relies on IR.
Here we investigated the effect of low doses of X-irradiation (< 0.1 Gray)
on paediatric dental stem cells including dental pulp stem cells from deciduous
teeth, dental follicle stem cells and stem cells from the apical papilla. DNA double
strand break (DSB) formation and repair kinetics were monitored as well as cell
cycle progression and cellular senescence.
Exposure to low doses of X-rays induces DNA DSBs as early as 30 minutes
post-irradiation. The number of DSBs returned to baseline levels 24 hours after
irradiation. Cell cycle analysis revealed marginal effects of IR on cell cycle
progression, although a slight G2/M phase block was seen in dental pulp stem cells
from deciduous teeth 72 hours after irradiation. Despite this cell cycle block, no
radiation-induced senescence was observed.
In conclusion, low IR doses were able to induce significant increases in the
number of DNA DSBs, but cell cycle progression seems to be minimally affected.
This highlights the need for more detailed and extensive studies on the effects of
exposure to low IR doses on different mesenchymal stem cells.
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ray exposure
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5.2 Introduction
Mesenchymal stem cells (MSCs) are of paramount importance for tissue
homeostasis which are potentially important targets of ionizing radiation (IR)
exposure. They can accumulate genotoxic damage following IR exposure, which
is either repaired efficiently, or they can accumulate irreversible damage. This
irreversible damage can trigger apoptosis or senescence, or unrepaired DNA
damage can persist and could lead to malignant transformation of the stem
cells.(1) Changes in the functionality of MSCs could be considered a predictive
indicator for future health hazards.(2, 3)
In 2000, Gronthos et al. identified and isolated odontogenic progenitor cells
from the dental pulp from adult patients.(4) These cells were dubbed dental pulp
stem cells (DPSCs). In the following years, several more types of dental stem cells
were described, such as the dental follicle stem cells (DFSCs), stem cells from the
apical papilla (SCAPs), pulp stem cells from human exfoliated deciduous teeth
(SHEDs), and periodontal ligament stem cells (PDLSCs).(5-8) An overview of these
cells and their potential use in dentistry is described by Bansal and Jain (2015).(9)
Today, one of the greatest challenges in radiation protection is unravelling
the potential detrimental effects of exposure to low doses of IR (below 100
milliGray (mGy)). This is important because people are exposed to low dose IR on
a daily basis, either from natural sources, or from man-made sources, such as
medical diagnostics.(10) Although there are epidemiological data on exposure to
doses higher than 100 mGy, i.e. high IR doses (e.g. from atomic bomb survivors,
medically and occupationally exposed populations and environmentally exposed
groups), no conclusive data exists on exposure to low doses of IR.(11) Currently,
risk estimation for low dose exposure is based on linear extrapolation from these
high dose data. This is the famous linear-no-threshold (LNT) model.(12-14) The LNT
model assumes that there is a linear relationship between IR dose and the
excessive cancer risk. When applying the LNT model, the following is assumed:
1) that there is a linear relationship between IR dose and the amount of radiation-
induced DNA double strand breaks (DSB), 2) that each DNA DSB has the
probability of inducing cellular transformations, and 3) that each transformation
has the same probability of resulting in carcinogenesis.(15) However, in the low
dose range (< 100 mGy), other phenomena than a linear response can occur.
There is evidence that low doses of IR could have beneficial effects, such as
hormesis and adaptive responses.(16, 17) Hormesis occurs when exposure to low IR
doses produces a favourable effect, whereas high IR doses result in detrimental
effects.(18) Adaptive responses occur when a very low dose, or priming dose,
stimulates cells which results in increased resistance to a second, larger dose of
the same trigger at a later time point. This could include the activation of genes
Chapter 5: in vitro assessment of the DNA damage response in dental stem cells following low dose X-
ray exposure
129
associated with DNA damage repair, stress scavenging, cell cycle control and
apoptosis.(16, 17)
DNA DSBs are the most crucial DNA lesions that are associated with
increased cancer risk and IR exposure. If not repaired correctly, DSBs can cause
genomic instability, mutations, chromosome aberrations and translocations, and
cell death.(19-22) To protect the DNA against these types of damage, eukaryotes
have developed the DNA damage response (DDR).(21, 22) In short, cellular
responses to IR-induced DNA DSBs are triggered by the activation of the ataxia
telangiectasia mutated (ATM) kinase. The phosphorylation of histone H2AX on
serine 139 (γH2AX) in the vicinity of the DNA DSB is one of the earliest ATM-
dependent responses.(20, 23, 24) γH2AX forms so called DNA damage foci in the
nucleus, or in the case of IR-induced DNA damage ‘IR-induced foci’ (IRIF). In
general, IRIF are distinct sub-nuclear structures to which the DDR proteins re-
localize. After phosphorylation, γH2AX initiates a signalling cascade leading to the
recruitment of multiple DDR proteins, including tumour suppressor p53-binding
protein 1 (53BP1).(19, 21, 25, 26)
53BP1 is a known DNA DSB sensor and a mediator and effector in the DDR
to DSBs.(21, 27, 28) Similar to γH2AX, 53BP1 has several functions in the DDR, such
as recruitment of DSB repair proteins, checkpoint signalling, determining the DSB
repair pathway and synapsis of distal DNA ends during non-homologous end-
joining (reviewed in Panier and Boulton).(27)
Evidence shows that both γH2AX and 53BP1 show a quantitative
relationship between the number of foci and the number of DNA DSBs.(21, 26, 29, 30)
Although γH2AX is a powerful tool to monitor DNA DSBs, artefacts do occur even
in the absence of DSBs.(22) Both γH2AX and 53BP1 foci can be visualized using
immunofluorescence microscopy and are detectable within minutes following
exposure to IR.(26, 31) Therefore, using an immunostaining protocol for
simultaneous detection of γH2AX and 53BP1 allows for better estimation of the
amount of DSBs present and reduces the impact of artefacts, since it is known
that γH2AX and 53BP1 co-localize in IRIF.(21, 32, 33)
DNA DSB could be efficiently repaired by the DDR, however, DNA DSBs
could persist. This could lead to cell cycle arrest, premature cellular senescence,
or apoptosis. As part of the DDR, cells halt their passage through the cell cycle,
allowing DDR proteins to repair DNA damage. If this damage persists, the cell
cycle could be irreversibly blocked. This cell cycle arrest can occur in all phases of
the cell cycle, but it was found that most cells are most sensitive to IR-induced
DNA damage in the G2/M phase.(34-36) Cellular senescence is a state of irreversible
growth arrest. This growth arrest occurs in the G1 phase of the cell cycle, therefore
cellular senescence is linked with changes in cell cycle progression. A hallmark of
senescent cells is the increased β-galactosidase activity in comparison to normal
cells. This can be detected by the so-called X-gal assay, which is considered as
the gold standard for senescence testing.(37, 38) Senescent cells also display a
senescence-associated secretory phenotype (SASP), which consists of several
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ray exposure
130
chemokines, cytokines, and regulatory factors. Some of these SASP factors are
linked with IR exposure, such as IL-6, IL-8, IGFBP-2 and IGFBP-3.(39, 40) IL-6 and
IL-8 interact with their surface receptors, which initiates several intracellular
pathways. Besides that, they can both induce or reinforce senescence in damaged
cells in a paracrine/autocrine manner.(39, 40) IGFBP-2 and IGFBP-3 interact with
insulin-like growth factor (IGF). They sequester IGF so it cannot bind to its
receptor, which eventually leads to inhibition of cell proliferation.(41) It is known
that premature cellular senescence can be caused by several stresses, such as
(persisting) DNA damage or reactive oxygen species.(42) It has been reported
before that exposure to (high) IR doses can cause premature cellular senescence.
This was observed both in mesenchymal stem cells and normal tissue cells.(43-48)
For low doses of IR, data is more scarce.(3, 49) Besides senescence, quiescence is
also an important process in stem cells. Quiescence is characterized by a cell cycle
arrest in the G0 phase. This phase is similar to the G1 phase, however cells do not
progress into the S phase. Unlike senescence, quiescence is a state of reversible
growth arrest. Quiescence occurs in cells that require a strict proliferation regime,
such as stem cells. It allows stem cells to assure genomic integrity until they are
needed for tissue repair, which is when they are stimulated to reprise the normal
cell cycle.(50) Evidence on the effects of IR on quiescence in mesenchymal stem
cells are scarce.(51, 52) Finally, cells can undergo apoptosis or programmed cell
death. Like premature cellular senescence, it is a response to extensive cellular
stress and mostly occurs when DNA damage repair is slow and/or incomplete.(53)
The aim of this study is to investigate the effects of low dose X-ray exposure
(< 100 mGy) on SHED, DFSCs, and SCAPs extracted from pediatric patients. DNA
DSB formation and repair, cell cycle progression, cellular quiescence, and cellular
senescence were monitored at several time points after exposure. Our data
present evidence that, although low doses of IR induce significant amounts of DNA
DSBs, DNA damage is effectively repaired and does not affect cell cycle
progression, nor induces premature cellular senescence in dental stem cells.
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5.3 Material and methods
5.3.1 Culturing dental stem cells
Three types of dental stem cells were used in this experiment: dental pulp
stem cells from deciduous teeth (SHED), dental follicle stem cells (DFSC) and stem
cells from the apical papilla (SCAP). These cells were extracted from teeth as
previously described.(4, 7, 8, 54) First, teeth were decontaminated using a povidone-
iodine solution. Second, they were sectioned and exposed pulp tissues were
collected. Third, these tissues were enzymatically digested using a type I
collagenase and dispase solution. Finally, the cells were ready to be cultured. After
extraction, the cells were seeded at a density of 104 cells per cm². They were
grown in Dulbecco’s Modified Eagle Medium (DMEM) containing 1 g/l D-glucose,
GlutaMAXTM and 10% foetal bovine serum (FBS) at 37° C with 5% CO2 in a
humidified incubator. The medium was refreshed every 2 – 3 days. At 70% - 80%
confluence the cells were passaged and seeded again at 104 cells per cm², or
frozen in liquid nitrogen for later use. To be sure that the stem cells keep their
phenotype, all stem cells were used between passages 1 and 5. Once enough cells
were obtained they were seeded either into 8-chamber Labtek® II slides at 2 x
104 cells per well or in 24-well plates at 4 x 104 cells per well (Greiner Bio-One,
Frickenhausen, Germany) 24 hours before irradiation. Six wells in each Labtek®
were used, resulting in six technical replicates. Each Labtek® represented one time
point per dose. In the 24-well plates cells were seeded in triplicates. For each cell
type, cells from three donor children were used (Table 5.1).
Table 5.1: Overview of dental stem cell donors
Age Gender
Donor 1 12 Male
Donor 2 11 Female
Donor 3 8 Female
5.3.2 X-irradiation conditions
The irradiation of samples was performed at the Laboratory for Nuclear
Calibrations (LNK) of the Belgian Nuclear Research Centre (SCK•CEN). In this
experimental design, it is of importance to mimic commercially available CBCT
devices as closely as possible. To this end X-rays with RQR9 beam quality, as
defined in the ISO 4037 standard, were used since RQR9 beam quality can be
used to simulate entrance beams used in diagnostic radiology. This beam quality
is created on the XStrahl 320 kV tube of LNK. The X-ray tube used a tube voltage
of 120 kiloVolt and a current of 1.8 milliAmpere. The distance between the focal
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132
point and the sample center was 100 cm. The X-ray beam was oriented vertically.
The inherent filtration was achieved by 3 mm of Be. Additional filtration was done
with 2.9 mm of Al and a dose area product monitor ionization chamber. The beam
diameter defined as Full Width at Half Maximum was 31 cm. The secondary
standard air kerma measurements are traceable to international standards, in
accordance with the ISO 17025 accreditation of LNK. The samples are always
smaller than the beam diameter. Using these parameters low doses and low dose
rates can be achieved which allows the simulation of diagnostic examinations.
Using a dose rate of 900 mGy per hour the samples were irradiated with doses of
100 ± 1.9 mGy, 50 ± 0.9 mGy, 20 ± 0.38 mGy, 10 ± 0.19 mGy and 5 ± 0.10
mGy. Control (0 mGy) samples were transported to the irradiation facility, but
they were not exposed to the radiation field (sham-irradiation).
5.3.4 Immunocytochemical staining for γH2AX and 53BP1
At specific time points after irradiation exposure (0.5, 1, 4 and 24 hours)
the culture medium was removed from the LabteksTM (NuncTM, ThermoFisher
Scientific, Waltham, MA, USA). Then the cells were washed twice using 1x
phosphate buffered saline (PBS). After washing, they were fixed in 2%
paraformaldehyde (PFA) in 1x PBS for at least 15 minutes at room temperature
(RT). Next the PFA was removed and the cells were washed twice with 1x PBS.
Fixed stem cells were double stained for γH2AX and 53BP1, both markers
for DNA DSBs. The 1x PBS was removed and then the cells were permeabilized
by incubating them in 0.25% Triton X-100 in 1x PBS for 3 minutes at RT. Then
the cells were washed three times in 1x PBS on a rocking platform. Next the cells
were blocked in pre-immunized goat serum (PIG). The PIG was diluted (1:5) in
Tris-HCl – NaCl blocking buffer (50 mM Tris-HCl, 150 mM NaCl, 0.1% Tween 20,
0.5% blocking reagent (FP1012, Perkin Elmer)) (TNB). The cells were blocked for
one hour at RT on a rocking platform, during which the primary antibody solution
was prepared. Primary antibodies were diluted in TNB, the mouse anti-human
γH2AX monoclonal antibody (05-636, Millipore, Massachusetts, USA) was diluted
1:300 and the rabbit anti-human 53BP1 polyclonal antibody (NB100-304, Novus
Biological, Abingdon, UK) was diluted 1:1000. After blocking, the cells were
incubated with the primary antibody solution for 1 hour at 37° C on a rocking
platform. After incubation, the cells were washed three times using 1x PBS. Next
the secondary antibody solution was prepared. An Alexa fluor 488-labelled goat
anti-mouse antibody (A11001, Life Technologies, Oregon, USA) and an Alexa fluor
568-labelled goat anti-rabbit antibody (A11011, Life Technologies, Oregon, USA)
were diluted 1:300 and 1:1000 in TNB, respectively. The cells were incubated with
the secondary antibody solution for another hour at 37° C on a rocking platform.
After this final incubation step, the cells were washed twice using 1x PBS. Next
the chambers were removed from the Labteks®. Then the samples were mounted
using Prolong® Diamond Antifade Mountant with 4',6-diamidino-2-phenylindole
Chapter 5: in vitro assessment of the DNA damage response in dental stem cells following low dose X-
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(DAPI) (P36962, Molecular ProbesTM by Life Technologies, Oregon, USA) as
nuclear counter stain. After mounting, the samples were stored at -20° C until
imaging.
Images were acquired with a Nikon Eclipse Ti fluorescence microscope using
a 40x dry objective (Nikon, Tokyo, Japan). Per technical replicate (n = 6 = number
of chamber of a LabtekTM used) at least 250 cells were counted. Afterwards, the
images were analysed using Fiji open source software.(55) Fiji allows for analysis
of each separate nucleus based on the DAPI signal. Within each nucleus, the
intensity signal for the Alexa fluorophores were analysed, after which the number
of co-localized γH2AX and 53BP1 foci per nucleus were determined in a fully
automated manner by using the Cellblocks tool.(56)
5.3.7 Cell cycle analysis
Cell cycle analysis was performed 1 h, 4 h, 24 h, and 72 h after X-irradiation
as described before.(43) In short, dental stem cells were treated with 10 µM of
BrdU for 1 hour. Afterwards, the cells were fixed with ice-cold 70% ethanol and
stored for a minimum of 24 hours. Next, the cells were permeabilized and stained
with rat anti-BrdU antibody, diluted 1 in 600 (AB6326, Abcam, Cambridge, UK).
They were also stained with 10 µg/ml of a 7-amino-actinomycin D (7-AAD)
solution (Sigma-Aldrich). Samples were analysed on a BD Accuri C6 flow
cytometer, with a maximum flow speed of 300 events per second. At least 20,000
cells were counted per sample.
5.3.8 Quiescence assay
G0 phase cells were identified 1 h, 4 h, 24 h, and 72 h after X-irradiation
using a quiescence assay. Dental stem cells were fixed with ice-cold 70% ethanol
following X-irradiation. Next, the cells were washed twice with 5% FBS (Gibco,
Massachusetts, USA) and 0.25% Triton X-100 (Sigma-Aldrich, Missouri, USA) in
1x PBS (PFT). Next, the cells were stained in PFT with 10 µg/ml 7-AAD (A9400-
1MG, Sigma-Aldrich, Missouri, USA) and 0.4 µg/ml pyronin Y (83200-5G, Sigma-
Aldrich, Missouri, USA) for 20 minutes at RT. Samples were analysed on a BD
Accuri C6 flow cytometer, with a maximum flow speed of 300 events per second.
At least 20,000 cells were counted per sample.
5.3.9 Β-galactosidase assay
Senescence was assessed 1, 3, 7, and 14 days after X-irradiation using the
senescence-associated β-galactosidase assay (ab65351, Abcam, Cambridge,
UK).(38) Cells were fixed for 15 minutes at RT using the fixative solution provided
with the kit. Next the cells were washed twice with 1x PBS. Then, the cells were
stained with 1 mg/ml X-gal solution at 37° C for 18 hours. Afterwards, the staining
Chapter 5: in vitro assessment of the DNA damage response in dental stem cells following low dose X-
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134
was stopped by adding 1 M Na2CO3. Next, the cells were incubated for 1 hour at
RT with a Giemsa dye, diluted 1:50 in 0.2 M acetate buffer (pH = 3.36). Finally,
the cells were washed twice with Milli-Q water and allowed to air dry. At least 300
cells per sample were analysed using a Nikon Eclipse Ti bright field microscope
using a 5x dry objective (Nikon, Tokyo, Japan).
5.3.10 Enzyme-linked immunosorbent assay (ELISA): IL-6, IL-8, IGFBP-
2, and IGFBP-3
For senescence assays on cytokine secretion, supernatant was collected 1,
3, 7 and 14 days following irradiation. Dental stem cells were grown in 12-well
plates. 1 ml of medium was collected at each time point. These samples were
used for the ELISA for the detection of IL-6, IL-8, IGFBP-2 and IGFBP-3. ELISA
was performed following manufacturer’s instructions (DY206, DY208, DY674, and
DY675, R&D Systems). Briefly, 96-well plates were coated overnight with a
capture antibody. Next, the wells were washed with washing buffer. Blocking
buffer was added and the plate was incubated for 1 hour at RT. After blocking,
the plate was washed once with washing buffer. Next, the supernatant was added
and incubated for 2 hours at RT. The plate was washed again, after which the
detection antibodies were added and the plate was incubated for 2 hours at RT.
Next, the plate was washed with washing buffer and a streptavidin-horse radish
peroxidase-labelled antibody was added and the plate was incubated for 20
minutes in the dark at RT. Then, the plate was washed with washing buffer. Next,
the substrate solution was added and the plate was incubated for 20 minutes in
the dark at RT. Afterwards, 2 M H2SO4 was added to stop the substrate reaction.
The optical density was measured at 450 nm and 570 nm using a
spectrophotometer (CLARIOstar, BMG Labtech, Offenburg, Germany).
5.3.11 Statistical analysis
Statistical analyses were performed using GraphPad Prism 8.0.0 (GraphPad
Software Inc., San Diego, USA). Graphs show mean ± standard error of the mean.
Two-way analysis of variance followed by post-hoc tests was performed to analyse
both time- and dose-dependent effects. P < .05 was considered statistically
significant.
Chapter 5: in vitro assessment of the DNA damage response in dental stem cells following low dose X-
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5.4 Results
5.4.1 Exposure to low doses of X-rays induces DSBs and activates the
DNA damage response in dental stem cells
DNA DSB formation and repair kinetics were monitored in dental stem cells
(SHED, DFSC, and SCAP), that were isolated from children, by microscopic
analysis of co-localized γH2AX and 53BP1 foci (N = 3). The number of co-localized
foci was determined 30 minutes, one hour, four hours and 24 hours after X-
irradiation with 0, 5, 10, 20, 50, and 100 mGy (Figure 5.1). The number of co-
localized foci increased with increasing radiation dose. Typically, the peak
response was seen between 30 to 60 minutes post-irradiation. After this period,
the number of foci decreased until baseline levels were reached 24 hours after
exposure. More specifically, in SHED, exposure to 100 mGy induced significantly
more co-localized foci 30 minutes and 1 h after irradiation compared to control
cells (0 mGy) (P < .0001). A dose of 50 mGy also resulted in more co-localized
foci 1 h after irradiation compared to 0 mGy (P = .0303). In the SCAPs, the
number of co-localized foci, observed after exposure to 100 mGy, was significantly
increased compared to 0 mGy 30 min, 1 h and 4 h after irradiation (P < .0001, P
< .0001, P = .0267, respectively). Furthermore, compared to control samples, 50
mGy irradiated samples showed more foci 30 min and 1 h p.i (P = .0018, P =
.0004, respectively) and 20 mGy irradiated samples showed more foci 1 h after
irradiation (P = .0416). In DFSC, more γH2AX and 53BP1 co-localized foci were
observed 30 min, 1 h and 4 h after exposure to 100 mGy (P < .0001, P < .0001,
P = .0374, respectively). 30 min and 1 h after exposure to 50 mGy and 30 minutes
after exposure to 20 mGy the amount of co-localized foci was increased as well in
DFSC (P < .0001, P = .0015, P = .0030, respectively). Furthermore, linear
regression plots show a linear dose response 30 min, 1 h and 4 h after irradiation.
Moreover, the slope decreased over time returning to a constant basal response
24 h after irradiation. Our linear regression analysis also resulted in a slope of
about 0.020 DNA DSBs per mGy (Table 5.2). No difference in radiation sensitivity
was observed between the different stem cell types.
Chapter 5: in vitro assessment of the DNA damage response in dental stem cells following low dose X-
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136
Figure 5.1. DNA double strand
break formation and repair
kinetics. A. Dental pulp stem cells
from deciduous teeth show a
significantly increased number of DNA
double strand breaks following
irradiation with 50 mGy and 100 mGy
30 min and 1 h after radiation
exposure. B. The number of co-
localized foci, observed after exposure
to 100 mGy, was significantly increased
compared to 0 mGy 30 min, 1 h and 4
h after irradiation (P < .0001, P <
.0001, P = .0267, respectively). 50
mGy irradiated samples showed more
foci 30 min and 1 h p.i (P = .0018, P =
.0004, respectively) C. In DFSC, more
foci were observed 30 min, 1 h and 4 h
after exposure to 100 mGy (P < .0001,
P < .0001, P = .0374, respectively). 30
min and 1 h after exposure to 50 mGy
and 30 minutes after exposure to 20
mGy the amount of co-localized foci
was increased as well in DFSC (P <
.0001, P = .0015, P = .0030,
respectively). The number of foci
returns to control levels 24 h after
irradiation. D-G. Representative image
taken 60 minutes after irradiation with
100 mGy. The nucleus (D.) shows five
clear γH2AX (E.) and 53BP1 (F.) foci,
which co-localize (G.) perfectly. *: P ≤
.05; **: P ≤ .0021; ****: P < .0001
D. E.
F. G.
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Table 5.2: Linear dose response relationship of co-localized γH2AX and 53BP1 foci
in dental stem cells
5.4.2 Cell cycle progression is not influenced by low doses of X-rays in
dental stem cells
Analysis of the percentage of cells that reside in a specific phase of the cell
cycle has revealed that exposure to low doses of X-rays does not induce major
cell cycle changes in dental stem cells (SHEDs and SCAPs) (N = 3 for each cell
type). Except for a slightly reduced number of G1/G0 phase cells 72 h after
irradiation in SHED (P = .019) and a slight increase in G2/M phase cells 72 h after
irradiation in SHED (P = .040) following a dose of 100 mGy, no changes were
observed (Figure 5.2). We did observe that the amount of G1/G0 phase cells
increases over time, whereas the amount of S- and G2/M phase cells decreases
over time, with almost no more cells in the S-phase after 72 h.
Cell type Time after irradiation
Slope (foci/mGy)
R² value P value
Dental pulp stem cells from deciduous teeth (SHEDs)
30 minutes 0.020 0.97 0.0003
1 hour 0.022 0.99 < 0.0001
4 hours 0.008 0.96 0.0005
24 hours -0.002 0.18 0.40
Dental follicle stem cells (DFSCs)
30 minutes 0.026 0.99 < 0.0001
1 hour 0.020 0.91 0.003
4 hours 0.008 0.75 0.025
24 hours -0.0001 0.013 0.83
Stem cells from the apical papilla (SCAPs)
30 minutes 0.019 0.98 0.0002
1 hour 0.022 0.99 < 0.0001
4 hours 0.009 0.94 0.0012
24 hours 0.005 0.47 0.13
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Figure 5.2. Cell cycle analysis of dental pulp stem cells from deciduous teeth. Dental
pulp stem cells from deciduous teeth (SHEDs) show a significantly decreased number of
G1/G0 phase cells 72 hours following X-irradiation with 100 mGy. Coincidently, a significant
increase in the number of G2/M phase cells was observed. *: P ≤ .05
5.4.3 Low dose X-irradiation rapidly decreases the amount of quiescent
cells
The effect of exposure to low doses of X-rays on cellular quiescence,
determined by measuring the percentage of G0 phase cells, was most pronounced
1 h after irradiation with 100 mGy. This was observed in SHEDs and SCAPs (N =
3). However, SHEDs showed still significant dose-dependent decreases in the
percentage of quiescent cells 4 h and 72 h after irradiation (Figure 5.3 and Table
5.3). In SCAPs, only a decrease was seen 1 h after irradiation with 100 mGy (P =
.030). It was also observed that the number of G0 decreased significantly over
time (Figure 5.3 and Table 5.3).
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Figure 5.3. Dose response of the percentage of G0 phase dental pulp stem cells
from deciduous teeth and stem cells from the apical papilla following low dose X-
irradiation. The percentage of G0 phase cells is plotted against the time after X-irradiation.
Significances are summarized in the table 5.3.
Table 5.3: Significant differences in the percentage of quiescent cells in dental
stem cells
Comparison Dental pulp stem cells from deciduous teeth
P value
Stem cells from the apical papilla
P value
1 h:CTRL vs. 50 mGy 0.0107 N.A.
1 h:CTRL vs. 100 mGy <0.0001 0.0296
1 h:20 mGy vs. 100 mGy 0.0011 N.A.
4 h:CTRL vs. 50 mGy 0.0072 N.A.
4 h:CTRL vs. 100 mGy 0.0064 N.A.
72 h: CTRL vs. 100 mGy 0.0025 N.A.
72 h:20 mGy vs. 100 mGy 0.0145 N.A.
5.4.4 Low dose radiation does not induce premature senescence in dental
stem cells
ELISA for SASP markers IL-6, IL-8, IGFBP-2, and IGFBP-3 showed no signs
of radiation-induced premature cellular senescence in SHEDs, DFSCs, and SCAPs
up to 14 days after exposure (N = 3 for each cell type). Although the values for
IL-6 and IL-8 in SHEDs increased significantly 14 days after irradiation exposure,
this was mostly due to the time in culture, rather than a radiation-induced effect
(Ptime = .006 and Ptime = .004, respectively). Levels of IGFBP-2 in SHEDs showed
changes over time, but overall there was a decreasing trend, which was not
influenced by radiation dose (Ptime = .022). Finally, in SHEDs, IGFBP-3 showed a
time dependent increase (Ptime = .005) (Figure 5.4).
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Figure 5.4. Senescence-associated secretory phenotype (SASP) protein secretion
in dental pulp stem cells from deciduous teeth (SHEDs) following low dose ionizing
radiation exposure The amount of interleukins (IL)-6 and IL-8, as well as the levels of
insulin-like growth factor binding proteins (IGFBP)-2 and IGFBP-3 indicate that there no
effect of low doses of ionizing radiation on the SASP. Two-way analysis of variance shows
that time after exposure is the major contributor to the observed effects (e.g. for IGFBP-2:
Ptime = .023).
The data from SASP markers were confirmed by the β-galactosidase
assay.(38) Data from dental stem cells show that there is an increase in the
percentage of senescent cells, but this increase is time-dependent. Low dose
radiation exposure does not induce cellular senescence in SHEDs, DFSCs, and
SCAPs (N = 3 for each cell type)(Figure 5.5).
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Figure 5.5. β-
galactosidase assay in
dental stem cells. The
percentage of senescent
cells indicates that low
doses of ionizing radiation
do not induce premature
cellular senescence. Two-
way analysis of variance
shows that time after
exposure is the major
contributor to the
observed effects (Ptime <
.0001 for all cell types).
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5.5 Discussion
Determining the biological effects of low dose IR exposure currently is the
greatest challenge in radiation protection. We aimed to investigate the DDR and
its consequences in human dental stem cells (i.e. SHEDs, DFSCs and SCAPs) after
exposure to X-ray doses below 100 mGy. SHEDs, DFSCs, and SCAPs are MSCs,
which are adult stem cells which can be isolated from human teeth. MSCs support
the maintenance of other cells, and the capacity of MSCs to differentiate into
several cell types makes these cells unique and full of possibilities.(57) Therefore,
maintaining the genetic stability of MSCs is of paramount importance. MSCs can
accumulate genotoxic damage following IR exposure, which is either repaired
efficiently, or they can accumulate irreversible damage. This persisting damage
could lead to malignant transformation of the stem cells.(1)
The formation and repair kinetics of DNA DSBs was monitored via
γH2AX/53BP1 immunostaining. Additionally, the impact of low dose radiation on
cell cycle progression, cellular quiescence and premature cellular senescence were
investigated. We report a significant increase in the amount of DNA DSBs 30
minutes and 1 hour after IR exposure. Repair kinetics clearly showed that the
number of DSBs in dental stem cells returned to baseline levels 24 hours after IR
exposure. Furthermore, a slight G2/M phase arrest was seen 72 hours after
irradiation in SHEDs, but not in SCAPs. Next, IR exposure resulted in reduced
levels of G0 cells in SHEDs and SCAP. However, in SCAP the decrease was only
statistically significant 1 h after irradiation and only for irradiation with 100 mGy.
For SHEDs, on the other hand, also 4 h and 72 h after irradiation a statistically
significant decrease was observed. Finally, low dose X-ray exposure did not result
in radiation-induced premature senescence in SHEDs, DFSCs, and SCAPs.
It is well-known that exposure to X-rays can induce DNA DSBs, which are
considered very harmful because unrepaired DSBs could result in mutations,
chromosome rearrangements/aberrations, and loss of genetic information.(25, 58-
60) Our results show that exposure to low dose IR, with a relatively high dose rate
of 0.9 Gy/h, induces significant increases in the number of DNA DSBs in dental
stem cells 30 - 60 minutes after irradiation.(61) Similar results have been reported
in human mesenchymal stem cells before.(3, 44, 62-66) However, some studies report
a persistent increase of γH2AX foci up to 48 hours after irradiation, which was not
observed in our study.(3, 62, 63) Linear regression analysis showed that the number
of DNA DSBs increases linearly with the IR dose. The slopes in SHEDs, DFSCs and
SCAPs ranged from 0.019 – 0.026 DNA DSBs per mGy. This is equivalent to 19 –
26 DNA DSBs per Gy, which is consistent with data published previously.(21, 67-70)
The formed DNA DSBs did not affect cell cycle progression in SCAPs, but we
did observe a slight G2/M phase block in SHEDs 72 hours following 100 mGy
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exposure. Although this increase was minimal, it was statistically significant. This
is in line with previous publications indicating that cells exhibit G2/M phase arrest
following exposure to high IR doses.(34-36, 44) However, there are data indicating
that exposure to high doses of IR results in G1 arrest in mesenchymal stem
cells.(64) Furthermore, the lack of cell cycle changes in SCAPs is in line with data
from Kurpinski et al. (2009), who also observed no changes in cell cycle
distribution in bone marrow mesenchymal stem cells following X-irradiation with
100 mGy.(71) Our data, taken together with data from literature, indicate that the
effect of X-irradiation on cell cycle progression is cell type-dependent.
Our cell cycle data reveal minimal changes in the G1/G0 phase of the cell
cycle. However, our data show for the first time a significant decrease in the
amount of quiescent or G0 phase cells in SHEDs 72 h after X-irradiation with 100
mGy. This would indicate that if the amount of G1/G0 phase remains constant, but
the amount of G0 phase cells decreases, that the amount of G1 phase cells increase
proportionally to the decrease of G0 phase cells. This indicates that low doses of
IR stimulate SHEDs to re-enter the cell cycle. It has been described that certain
extrinsic stresses such as IR-induced reactive oxygen species, which are
generated by radiolysis of water following IR exposure, can stimulate stem cell to
re-enter the cell cycle.(72) This could, at least partly, explain our observations.
Finally, we did not observe radiation-induced cellular senescence following
exposure to low doses of IR, except for SHEDs where a slight increase in G2/M
arrest was observed 72 hours after irradiation after irradiation with 100 mGy.
However, our data clearly showed time-dependent induction of senescence. This
was seen both in results from the X-gal assay, which is considered the gold
standard, as in analysis of the SASP. It has been reported before that high doses
of IR can induce cellular senescence in mesenchymal stem cells.(44-46, 48, 73)
However, evidence of low dose IR-induced senescence is scarce.(3, 74) These
studies contradict our data. On the other hand, there are studies that support our
findings.(63, 75) Due to these contradicting data and the fact that low dose
radiation-induced senescence is poorly investigated, it is impossible to conclude
at this time whether low doses of IR do cause cellular senescence in these cell or
not. More detailed studies on this matter are warranted.(10)
In conclusion, we found that exposure of dental stem cells to low doses of
X-rays results in the induction of DNA DSBs and that the number of DNA DSBs
increases linearly with the radiation dose. After 24 hours, these DNA DSBs are
efficiently repaired and returned to baseline levels. These observations are in line
with the LNT model which is currently applied in radiation protection. We report
for the first time, to the best of our knowledge, that exposure to low IR doses
results in an acute dose-dependent decrease in the number of quiescent SHEDs
and SCAPs, which is still observed 72 hours after X-irradiation in SHEDs. However,
we did not find adverse effects on cell cycle progression. No persistent cell cycle
changes, nor induction of premature cellular senescence were observed. Although
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this is in line with previous studies, there are also studies indicating that low doses
of IR can cause cell cycle arrest and senescence. We cannot conclude that there
is no threshold for the biological effects of IR exposure. Our data highlight the
need for more detailed and extensive studies on the effects of exposure to low
doses of IR.
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Chapter 6:
Antioxidant response in buccal
mucosa cells and saliva samples
following CBCT examination
Belmans N, Smeets K, Vermeesen R, Salmon B, Baatout S, Jacobs R., Lucas S,
Lambrichts I, Moreels M Antioxidant response in buccal mucosa cells and saliva
samples following CBCT examination. In preparation
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6.1 Introduction
Reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), the
hydroxyl radical (OH•), and the superoxide anion (O2•-), are formed by the partial
reduction of molecular oxygen (O2). Intracellular ROS are either generated
endogenously during the process of mitochondrial phosphorylation, or they form
following exposure to exogenous stimuli such as bacterial infections or ionizing
radiation (IR). When the amount of ROS exceeds the balancing capacity of the
intracellular antioxidants, which help regulate the cellular redox balance, an
imbalance between oxidants and antioxidants occurs, which is called ‘oxidative
stress’.(1, 2) Oxidative stress could result in ROS-mediated damage to nucleic acids,
lipids, and proteins. It has been linked to cardiovascular diseases,
neurodegeneration, carcinogenesis, diabetes, and aging.(3-7) However, besides its
involvement in pathogenesis of the aforementioned conditions, it has become
clear during the past 25 years that ROS also serve as important signalling
molecules that help regulate important biological and physiological processes,
such as cellular differentiation, tissue regeneration, and prevention of aging.(2, 8,
9) In short, redox biology largely depends on H2O2, whereas OH• and O2•- mostly
cause oxidative stress, in normal physiological conditions.(2) In non-physiological
conditions, however, ROS (especially OH• and O2•-) cause oxidative stress which
can lead to severe DNA damage, including DNA breaks, base damage, destruction
of sugars, cross links and telomere dysfunction.(10) This is the case with exposure
to IR, which results in the radiolysis of intra- and extracellular water molecules,
that in turn generates ROS.(11) IR-induced oxidative stress could, when the
damage is not repaired efficiently, lead to cell death or mutations that could result
in carcinogenesis. Of these IR-induced ROS, OH• and O2
•- are the most reactive
ones. (12, 13)
Oxidative DNA damage is widely accepted to contribute to cancer
development.(14, 15) However, oxidative DNA damage occurs continuously in vivo
at the guanine DNA base and is usually caused by OH•. Measuring these oxidative
DNA modifications could be potential biomarkers that predict cancer development
later in life.(16) 7,8-dihydro-8-oxo-2’-deoxyguanosine (8-oxo-dG) is the most
frequently measured oxidatively modified DNA base.(17, 18) It is so frequently
measured because there are sensitive detection techniques available, it is formed
by several important ROS including O2•- and OH•, and finally, it is a mutagenic
lesion. The latter entails that cells have mechanisms to identify the presence of
8-oxo-dG and that they will remove 8-oxo-dG via nucleotide/base excision repair.
8-oxo-dG has been successfully measured in blood, urine and saliva samples.(19-
25) It is known that 8-oxo-dG levels increase following high doses of IR.(26-30) We
previously demonstrated that 8-oxo-dG levels increased significantly in saliva
Chapter 6: Antioxidant response in buccal mucosa cells and saliva samples following CBCT examination
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samples of children 30 min following a cone-beam computed tomography (CBCT)
examination (see Chapter 4).
The intracellular antioxidant system is an important mechanism to maintain
intracellular redox homeostasis. It allows low concentrations of ROS to be present
within the cell, while preventing accumulation of high levels of ROS in normal
physiological conditions.(9) This delicate balance is essential for a normal cellular
function.(31) The redox balance is maintained by endogenous antioxidants,
including enzymatic antioxidants, hydrophilic antioxidants, and lipophilic radical
antioxidants, which all counteract the surplus of free radicals and neutralize
oxidants.(32) Superoxide dismutases (SOD), catalase (CAT), and glutathione
peroxidases (GSH-Px) are examples of enzymatic antioxidants. They have the
ability to decompose ROS.(33) SOD dismutates O2•- to H2O2 and O2.(34) H2O2 is in
turn neutralized by other enzymes such as CAT and GSH-Px.(35-39)
As discussed earlier, exposure to IR leads to oxidative stress through the
formation of ROS from radiolysis of water. It was shown that high doses of IR
significantly increase the gene expression and activities of SOD2, CAT, and GSH-
Px. However, it has been shown that after irradiation with a low IR dose cells are
primed for exposure to higher IR doses. Furthermore, these cells also show
increased gene expression for genes encoding for antioxidants. Primed cells, in
turn, show an increased antioxidant response in comparison to non-primed cells
following high IR dose exposure, resulting in a higher radioresistance in these
cells.(40, 41) Previously, we have demonstrated that the total antioxidant capacity
increased in saliva from children following CBCT examinations. Interestingly, an
opposite response was observed in adults, indicating potential age-dependent
differences in anti-oxidant capacities. (Belmans et al.(2019), submitted; see
chapter 4). Moreover, we showed that CBCT examinations increase the level of
oxidative damage (i.e. 8-oxo-dG levels) in saliva samples from children. The
changes in antioxidant capacity indicate that children and adults might respond
differently to low doses of IR.(42) To gain more insight into the antioxidant
response following CBCT examinations, we investigated the enzyme activity of the
two major endogenous antioxidants, i.e. SOD, and CAT, in saliva samples from
children following CBCT examination. Additionally the gene expression levels of
SOD1, CAT, and GPx1 were monitored in buccal mucosa cells (BMCs) from children
and adults following CBCT examination.
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6.2 Materials and methods
6.2.1 Patient selection
Patients with various indications were referred to the clinic for CBCT
examination. They were examined using CBCT device settings that match their
individual needs. Thus the field of view, tube voltage (kV), tube current (mAs)
and resolution mode are adjusted to fit with each individual’s indication and age,
as described in the DIMITRA position statement by Oenning et al. (2017).(43)
The study was performed following current General Data Protection
Regulation guidelines. Ethical approval was obtained at the Oral and MaxilloFacial
Surgery – Imaging & Pathology department (Katholieke Universiteit Leuven,
Leuven, Belgium) (B322201525196).
Eligible patients were children/adolescents from 3 to 18 years old, as well
as adults (> 18 years old), with good oral hygiene. Exclusion criteria were the
presence of systemic diseases, the use of antibiotics or anti-inflammatory drugs,
smoking and not giving informed consent prior to enrolment. In case of underage
children, both parents needed to consent unless one parent has explicit permission
from the other parent.(44)
6.2.2 Saliva collection
Saliva samples were collected according to the DIMITRA study protocol.(44)
In short, saliva samples were collected right before and 30 minutes after CBCT
examination using the passive drool method. Immediately after collection, the
whole saliva was stored at -20° C until shipment. After shipment to the lab, saliva
samples were centrifuged at 10,000 g at 4° C and the supernatant was stored at
-80° C until further analysis.
6.2.3 Buccal mucosa cell collection
The collection method was based on the protocol described in Belmans et
al. (2019).(44) Briefly, synthetic swabs were used to collect BMCs just before, 30
minutes, 24 hours and 48 hours after CBCT examination. Before each swab the
patient’s mouth was rinsed twice with water. After sample collection, the swabs
were transferred to tubes containing RNAprotect Cell Reagent (76526, Qiagen,
Hilden, Germany).
6.2.4 Enzyme activity assay
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The enzyme activity for both SOD and CAT were assessed using
commercially available kits (706002 and 707002 respectively; Cayman Chemical,
Michigan, USA). The assay kits were performed according to manufacturer’s
instructions. Briefly, standards/undiluted saliva samples were added to a 96-well
plate. Next, radical detector (SOD) or assay buffer (CAT) was added to the wells.
The enzymatic reactions were initiated by adding xanthine oxidase (SOD) or H2O2
(CAT). For the CAT assay, additional potassium hydroxide needs to be added.
After incubation, the CAT reaction is stopped by adding potassium periodate and
then the absorbance was read at 440 nm (SOD) or 540 nm (CAT) with a microplate
reader (Clariostar, BMG Labtech, Ortenberg, Germany).
6.2.5 RNA isolation from RNAprotect Cell Reagent
For RNA isolation both TRIzolTM Reagent (15596026, InvitrogenTM, Carlsbad,
USA) and the Qiagen RNeasy Plus Micro kit (74034, Qiagen, Hilden, Germany)
were used. Briefly, the samples were centrifuged for 1 hour at 2000 g at 4° C.
Then the supernatant was removed and the pellet was lysed by adding 1 ml of
TRIzolTM Reagent. The cells were incubated for 30 minutes at 37° C to allow for
full cell lysis. Next, 200 µl of chloroform was added. Then, the samples were
shaken vigorously and incubated for 3 minutes at room temperature (RT). Next,
the samples were centrifuged at 12.000 g for 15 minutes at RT. Afterwards, the
aqueous phase was transferred to a 1.5 ml microcentrifuge tube. Then, an equal
amount of 70% ethanol (EtOH) was added to the sample after which the samples
were put on the RNeasy spin column and centrifuged for 30 seconds at 14.000 g.
Next, the RNeasy spin column was washed using RW1 buffer and the samples
were centrifuged for 30 seconds at 14.000 g. Then the column was washed with
RPE buffer and centrifuged for 30 seconds at 14.000 g. After this wash step, the
column was washed with 80% EtOH and centrifuged for 2 minutes at 14.000 g.
After washing, the column was put on a new 1.5 ml microcentrifuge tube and 20
µl of RNase-free water was added to the column. The column was then centrifuged
for 1 minute at 14.000 g. The eluted RNA samples were stored on ice and RNA
concentrations were determined using a NanoDropTM 2000c (ThermoFisher
Scientific, Waltham, MA, USA). After the RNA concentration was determined, the
RNA samples were stored at -80° C.
6.2.6 cDNA synthesis
The Promega GoScriptTM Transcriptase kit (A2801, Promega Benelux N.V.,
Leiden, The Netherlands) was used for cDNA synthesis. In short, RNA samples
were thawed on ice. Then 300 ng of RNA was diluted to 14 µl using RNase-free
water in sterile polymerase chain reaction (PCR) tubes. The samples were
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centrifuged briefly. Then they were incubated for 5 minutes at 70° C. During
incubation, the reverse transcription mix was prepared. After incubation, the
samples were chilled on ice for at least 5 minutes. Next, 6 µl of reverse
transcription mix was added to each sample. Then the samples were mixed gently
and centrifuged briefly. The, the samples were incubated in a thermocycler
(Proflex PCR system, ThermoFisher Scientific, Waltham, MA, USA). First for 5
minutes at 25° C, then at 42° C for 60 minutes, and finally 15 minutes at 70° C.
Afterwards, the samples were diluted 1:3 in RNase-free water, resulting in a final
concentration of 5 ng/µl cDNA. Finally, the samples were stored at -20° C.
6.2.7 Gene expression analysis using TaqManTM probes and primers
Gene expression levels of SOD1, CAT, and GPx1 are assessed using
TaqManTM gene expression assays (Hs00533490_m1, Hs00156308_m1, and
Hs00829989_gH, respectively; ThermoFisher Scientific, Waltham, MA, USA).
PGK1 and GAPDH were used as reference genes (4333765F and Hs02786624_g1,
respectively; ThermoFisher Scientific, Waltham, MA, USA). These reference genes
were chosen based on previous data on patient samples following radiation
exposure.(45) In short, cDNA samples and primer/probe sets were thawed on ice.
A real-time polymerase chain reaction (qPCR) mastermix was prepared by diluting
20x primer/probe set and 2x TaqManTM Universal Mastermix II with Uracil-N-
glycosylase (Applied Biosystems, Foster City, CA, USA) in milliQ water. 15 µl of
qPCR mastermix is added per well. Next, 5 µl of cDNA, which equals 25 ng, is
added to each well. Then the samples are placed in the RotorGene Q series
(Qiagen, Hilden, Germany). Samples were analysed using following set-up: 1)
incubation at 50° C for 2 minutes followed by an incubation at 95° C for 10
minutes, 2) cycling between 15 seconds at 95° C and 60 seconds at 60 °C for 40
cycles. At the end of each incubation step at 60° C, a fluorescent signal was
acquired. QPCR data was analysed using the Pfaffl method.(46)
6.2.8 Dose calculations – Monte Carlo simulation
A fully validated Monte Carlo framework, which was developed by the
DIMITRA group, was used for dosimetric calculations.(47, 48) This Monte Carlo
simulation relies on a database of pediatric head voxel models.(49) By using this
Monte Carlo framework, absorbed organ doses were calculated for each individual
patient. When simulating organ doses, the normalized absorbed organ dose values
are provided in µGy/mAs. In this Monte Carlo framework, normalized absorbed
organ doses are related to the age of the patient via the following equation:
y = a x ln(x) + b
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where y is the normalized absorbed organ dose (µGy/mAs), x is the age of the
patient at the time of the scan, and the constants a and b are factors that depend
on the organ scanned, the clinical case, and the device used.(47) Simply multiplying
the normalized absorbed organ dose by the mAs used for each specific scanning
protocol results in an absorbed organ dose value. Thus the absolute organ dose
can be calculated as follows:
yi,j = [a x ln(x) + b] x mAsj
where i represents a specific organ, and j stands for a specific examination. Note
that this equation is not validated for adults, i.e. patients older than 18 years old.
Therefore, no doses were simulated for adults using this equation.
6.2.9 Statistical analysis
Statistical analysis was performed using GraphPad 8.00 (GraphPad Inc., CA,
USA). The results of the enzyme activity assays were analysed using two-tailed
paired t-tests. To analyse differences between boys and girls, two-tailed unpaired
t-tests were performed. For gene expression analysis, repeated measures one-
way analysis of variance was performed. All tests listed above are parametric
tests. If the conditions to test parametrically were not met, non-parametric
alternatives were used. P values lower than .05 were considered as significant.
Results are shown as mean ± standard error of the mean.
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6.3 Results
6.3.1 Patients and dose exposure
For this study, the aim is to include 50 children referred for CBCT and about
10 children referred for head and neck CT. For adults, 15 patients will be included
that were referred for CBCT as well as 10 that were referred for CT So far, 34
children and 20 adults are included in this study (Table 6.1). Note that not all
patients samples have been analysed at this point. Therefore, no dose calculations
are included.
Two CBCT devices were used, namely Accuitomo 170 (Morita, Osaka,
Japan) and NewTom VGi-evo (Cefla S.C., Imola, Italy). The study was approved
by the ethical committee of the participating hospital (see Material & Methods
section). All patients (or their parents, in case of children) gave written informed
consent.
Table 6.1: Overview of patients included in this study up to now
#
patients
# CBCT examinations
(included/foreseen)
# CT examinations
(included/foreseen)
Age (range)
Gender (m/f)
Children 34 34/50 -/10 7 - 17 16/18
Adults 20 12/15 8/10 18 - 84 10/10
6.3.1 CBCT examination leads to an increase in SOD activity which is
dependent on gender
Analysis of the SOD activity in saliva samples from children shows a
significant increase in SOD activity 30 minutes after CBCT examination. SOD
activity (U/ml) increases from 3.74 ± 0.55 U/ml at baseline to 5.95 ± 0.78 U/ml
30 minutes after CBCT examination (N = 32, p = .0052) (Figure 6.1).
Analysis based on gender revealed that the SOD activity increases
significantly in boys, but nog in girls (Figure 6.2). In boys, SOD activity increases
from 3.10 ± 0.53 U/ml to 6.11 ± 1.25 U/ml after CBCT (N = 14, p = .0067). In
girls, the SOD activity increases as well, from 4.24 ± 0.88 U/ml at baseline to
5.82 ± 1.03 U/ml 30 minutes after CBCT examination (N = 18; p = .13). Both at
baseline (p = .64) and after CBCT examination (p = .99), there is no difference
between boys and girls.
Chapter 6: Antioxidant response in buccal mucosa cells and saliva samples following CBCT examination
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Figure 6.1. Superoxide dismutase
(SOD) activity 30 minutes after
CBCT examination in saliva samples
from children. The SOD activity
increases significantly from 3.74 ± 0.55
U/ml before CBCT to 5.95 ± 0.78 U/ml
after CBCT (N = 32, p = .0052). * = p <
0.01
Figure 6.2. Gender differences in superoxide dismutase (SOD) activity after CBCT
examination in saliva samples from children. In boys, the SOD activity increases
significantly from 3.10 ± 0.53 U/ml before CBCT to 6.11 ± 1.25 U/ml after CBCT (N = 14,
p = .0067). In girls, the SOD activity varied from 4.24 ± 0.88 U/ml at baseline to 5.82 ±
1.03 U/ml after CBCT examination (N = 18, p = .13). At baseline (p = .64) and after CBCT
examination (p : .99) there is no statistical difference between boys and girls. **: p = .0067;
U = unit of enzyme catalytic activity (1 U = 1 µmol•min-1).
6.3.2 CBCT examination leads to an increase in CAT activity
Analysis of the CAT activity in saliva samples from children shows an
increase in CAT activity 30 minutes after CBCT examination. CAT activity
(nmol/min/ml) increases from 7.68 ± 0.76 nmol/min/ml at baseline to 9.45 ±
Chapter 6: Antioxidant response in buccal mucosa cells and saliva samples following CBCT examination
159
0.71 nmol/min/ml 30 minutes after CBCT examination (N = 32, p = .0014) (Figure
6.3).
Analysis based on gender revealed that the CAT activity increases
significantly in boys, but nog in girls (Figure 6.4). In boys, CAT activity increases
from 7.11 ± 1.11 nmol/min/ml to 9.88 ± 1.10 nmol/min/ml after CBCT (N = 14,
p = .017). In girls, the CAT activity increases as well, from 8.12 ± 1.06
nmol/min/ml at baseline to 9.11 ± 0.94 nmol/min/ml 30 minutes after CBCT
examination (N = 18; p = .12). Both at baseline (p = .46) and after CBCT
examination (p = .69), there is no difference between boys and girls.
Figure 6.3. Catalase (CAT) activity 30
minutes after CBCT examination in saliva
samples from children. The CAT activity
increases significantly from 7.68 ± 0.76
nmol/min/ml before CBCT to 9.45 ± 0.71
nmol/min/ml after CBCT (N = 32, p = .0014).
** = p < 0.002
Chapter 6: Antioxidant response in buccal mucosa cells and saliva samples following CBCT examination
160
Figure 6.4. Catalase (CAT) activity 30 minutes after CBCT examination in saliva
samples from boys and girls. In boys, the CAT activity increases significantly from 7.11
± 1.11 nmol/min/ml at baseline to 9.88 ± 1.10 nmol/min/ml after CBCT (N = 14, p = .017).
In girls, the CAT activity varied from 8.12 ± 1.06 nmol/min/ml at baseline to 9.11 ± 0.94
nmol/min/ml after CBCT (N = 18, p = .12). At baseline (p = .64) and after CBCT examination
(p : .69) there is no statistical difference between boys and girls. *: p = .017
6.3.3 Changes in SOD1, CAT, and GPx1 gene expression in children and
adults
Relative SOD1 gene expression changes statistically significantly after CBCT
examination in children (pANOVA = .03). The relative gene expressions decreases
from 0 ± 0.26 at baseline to -1.2 ± 0.41 30 minutes after CBCT examination (N
= 28; p = .01). (Figure 6.5 A). 48 h after CBCT examination, SOD1 gene
expression was still decreased in children, i.e. -0.98 ± 0.19 compared to baseline
(p = .0003). In adults, no significant changes were observed (N = 12; pANOVA =
.53) (Figure 6.5 B).
Next, in children the relative CAT gene expression does not change following
CBCT examination (N = 28)(Figure 6.5 C). Similarly, no relative gene expression
changes were observed in adults (N = 12; pANOVA = .22) (Figure 6.5 D).
Finally, the relative GPx1 gene expression changes statistically significantly
in children after CBCT examination (N = 28; pANOVA = .0002). Post-hoc testing
indicates that the relative gene expression decreases significantly from 0.00 ±
0.30 at baseline to -1.3 ± 0.35, 48 h after CBCT examination (N = 28; p = .007)
(Figure 6.5 E). In adults, a statistically significant changes occur in relative GPx1
gene expression (N = 12; pANOVA = .027) (Figure 6.5 F). Post-hoc testing indicates
that the relative gene expression decreases significantly from 0.00 ± 0.37 at
baseline to -1.4 ± 0.43, 48 h after CBCT examination (p = .007) (Figure 6.5 F).
Chapter 6: Antioxidant response in buccal mucosa cells and saliva samples following CBCT examination
161
Figure 6.5. Relative gene expression changes in the SOD1, CAT, and GPx1 genes in
children and adults. A. Relative SOD1 gene expression changes statistically significantly
30 min after CBCT examination in children (pANOVA = .03). The relative gene expression
decreases from -0.00 ± 0.26 at baseline to -1.2 ± 0.41 30 minutes after CBCT examination
(N = 28; p = .01). 48 h after CBCT examination, SOD1 gene expression was decreased in
children, i.e. -0.98 ± 0.19 compared to baseline (p = .0003). B. In adults, no statistically
significant changes were observed (N = 12; pANOVA = .53). C. The relative CAT gene
expression does not change following CBCT examination in children (N = 28). D. No changes
were observed in adults (N = 12; pANOVA = .22). E. The relative GPx1 gene expression
changes statistically significantly in children after CBCT examination (pANOVA = .0002). The
relative gene expression decreases significantly from 0.00 ± 0.30 at baseline to -1.3 ± 0.35
48 h after CBCT examination (N = 28; p = .007). F. In adults, a statistically significant
changes occurs in relative GPx1 gene expression (N = 12; pANOVA = .027). The relative gene
expression decreases statistically significantly from 0.00 ± 0.37 at baseline to -1.4 ± 0.43,
48 h after CBCT examination (p = .007) * = p < .05; **: p < .002*** = p < .0005
Chapter 6: Antioxidant response in buccal mucosa cells and saliva samples following CBCT examination
162
6.4 Discussion
Our preliminary data indicate that exposure to low doses of IR, such as
those associated with CBCT examinations, leads to an increase in SOD and CAT
activity in saliva samples from children 30 minutes after X-irradiation. These data
imply that in response to CBCT induced oxidative damage, as measured by
increased levels of 8-oxo dG (Belmans et al., submitted), the enzymatic activity
of SOD and CAT increases in an attempt to scavenge the additional ROS that is
formed due to low dose exposure. However, our gene expression data reveal that
after CBCT examination the relative gene expression of SOD1 and GPx1 decreases
in children. The relative gene expression for SOD1 statistically significantly
decreased 30 minutes after CBCT examination, and remained decreased 48 hours
after the CBCT examination. Finally the relative gene expression of GPx1
decreased only 48 h after CBCT examination in comparison to gene expression
levels at baseline. In adults, no statistically significant changes in relative gene
expression were observed, except for GPx1, where the relative gene expression
decreased significantly 48 hours after CBCT examination, as it did in children. For
CAT and GPx1 expression, children and adults reacted similarly. However, our
data indicate that they react differently when it comes to SOD1 expression after
CBCT examination. Children show a fast reduction in gene expression, whereas
no changes occur in adults.
It is known that low doses of IR can induce ROS scavengers.(50) In
occupational exposed staff, it has been shown that SOD activity is increased in
comparison to controls, however, the CAT activity was found to be reduced.(51)
Our data on enzyme activity are more in line with results from adaptive response
studies. These studies show that exposure to low doses of IR increases the activity
of antioxidant enzymes, which will protect the cells when they are exposed to a
subsequent high IR dose.(41) Surprisingly, our gene expression data indicate that,
although the enzyme activity increases, gene expression decreases, especially for
SOD1 (in children only) and GPx1. However, interpretation of these data should
be done with caution, since these results are only preliminary.
For both SOD and CAT activity, we observed that the increase in enzyme
activity was only statistically significantly increased in boys, not in girls. Gender-
related differences were previously shown by Eken et al. (2012), who
demonstrated that the SOD activity was significantly increased in exposed male
radiation-exposed hospital staff, but not in female radiation-exposed hospital
staff. They did not provide data on CAT activity and gender differences.(51) To the
best of our knowledge this is the first time that this is observed in young boys and
girls following CBCT examination.
It should be noted that these data are preliminary and show an acute
increase in SOD and CAT activity, since it was only determined 30 minutes
following CBCT examination. Data should also be collected at later time points to
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163
see if the increase is persistent, and thus improves the individual’s response to IR
exposure, or that the increase is transient, and thus is a temporary defence
mechanism countering ROS produced by the CBCT examination. Data on the
relative gene expression hints at either acute (i.e. SOD1) or delayed (i.e. GPx1)
decrease in gene expression levels after CBCT examination in children. In adults,
only a delayed decrease in GPx1 gene expression was observed. However, in this
experiment, the sample size was rather small. Therefore, more subjects will be
included. Additionally, as with the results presented in chapter 4, it would be
interesting to also include adults (for enzyme activity assays). That way age-
related differences could be studied as well. Furthermore, including adults for the
enzyme activity assays could also give more insight in the gender differences that
we have observed in young children. It would be interesting to see whether these
differences persist in adulthood, or that the gender difference disappears, or even
reverse, with increasing age.
To tackle the aforementioned issues, the Radiobiology Unit from the Belgian
Nuclear Research Centre (Mol, Belgium) and the Department of Dentomaxillofacial
Surgery and of Imaging and Pathology (KU Leuven, Leuven, Belgium) have
applied for a research grant from the ‘Fonds Wetenschappelijk Onderzoek
Vlaanderen’ (FWO). This FWO grant was awarded to the research groups and
provides funds to continue this project from 2018 until 2021 under grant number
G0A0918N.
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6.5 References
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15. Poulsen HE, Prieme H, Loft S. Role of oxidative DNA damage in cancer initiation and promotion. Eur J Cancer Prev. 1998;7(1):9-16. 16. Halliwell B, Gutteridge JMC. Measurement of reactive species. Free Radicals in Biology and Medicine: Oxford University Press; 2015. 17. Kasai H, Nishimura S. Hydroxylation of deoxyguanosine at the C-8 position by ascorbic acid and other reducing agents. Nucleic Acids Res. 1984;12(4):2137-45. 18. Kasai H, Nishimura S. Hydroxylation of deoxy guanosine at the C-8 position by polyphenols and aminophenols in the presence of hydrogen peroxide and ferric ion. Gan. 1984;75(7):565-6. 19. Cooke MS, Singh R, Hall GK, Mistry V, Duarte TL, Farmer PB, et al. Evaluation of enzyme-linked immunosorbent assay and liquid chromatography-tandem mass spectrometry methodology for the analysis of 8-oxo-7,8-dihydro-2'-deoxyguanosine in saliva and urine. Free Radic Biol Med. 2006;41(12):1829-36. 20. Evans MD, Saparbaev M, Cooke MS. DNA repair and the origins of urinary oxidized 2'-deoxyribonucleosides. Mutagenesis. 2010;25(5):433-42. 21. Rossner P, Jr., Mistry V, Singh R, Sram RJ, Cooke MS. Urinary 8-oxo-7,8-dihydro-2'-deoxyguanosine values determined by a modified ELISA improves agreement with HPLC-MS/MS. Biochem Biophys Res Commun. 2013;440(4):725-30. 22. Cooke MS, Evans MD, Dizdaroglu M, Lunec J. Oxidative DNA damage: mechanisms, mutation, and disease. FASEB J. 2003;17(10):1195-214. 23. Breton J, Sichel F, Pottier D, Prevost V. Measurement of 8-oxo-7,8-dihydro-2'-deoxyguanosine in peripheral blood mononuclear cells: optimisation and application to samples from a case-control study on cancers of the oesophagus and cardia. Free Radic Res. 2005;39(1):21-30.
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24. Tothova L, Kamodyova N, Cervenka T, Celec P. Salivary markers of oxidative stress
in oral diseases. Front Cell Infect Microbiol. 2015;5:73. 25. Arunachalam R. Salivary 8-Hydroxydeoxyguanosine – a valuable indicator for oxidative DNA damage in periodontal disease. The Saudi Journal for Dental Research. 2014;6:15-20. 26. Haghdoost S, Sjolander L, Czene S, Harms-Ringdahl M. The nucleotide pool is a significant target for oxidative stress. Free Radic Biol Med. 2006;41(4):620-6. 27. Shakeri Manesh S, Sangsuwan T, Pour Khavari A, Fotouhi A, Emami SN, Haghdoost S. MTH1, an 8-oxo-2'-deoxyguanosine triphosphatase, and MYH, a DNA glycosylase, cooperate to inhibit mutations induced by chronic exposure to oxidative stress of ionising radiation. Mutagenesis. 2017;32(3):389-96. 28. Hall J, Jeggo PA, West C, Gomolka M, Quintens R, Badie C, et al. Ionizing radiation biomarkers in epidemiological studies - An update. Mutat Res. 2017;771:59-84. 29. Haghdoost S, Czene S, Naslund I, Skog S, Harms-Ringdahl M, Haghdoost S, et al. Extracellular 8-oxo-dG as a sensitive parameter for oxidative stress in vivo and in vitro The nucleotide pool is a significant target for oxidative stress. Free Radic Res. 2005;39(2):153-62. 30. Haghdoost S, Svoboda P, Naslund I, Harms-Ringdahl M, Tilikides A, Skog S. Can 8-oxo-dG be used as a predictor for individual radiosensitivity? Int J Radiat Oncol Biol Phys. 2001;50(2):405-10. 31. Holmstrom KM, Finkel T. Cellular mechanisms and physiological consequences of redox-dependent signalling. Nature reviews Molecular cell biology. 2014;15(6):411-21. 32. Ratnam DV, Ankola DD, Bhardwaj V, Sahana DK, Kumar MN. Role of antioxidants in prophylaxis and therapy: A pharmaceutical perspective. J Control Release. 2006;113(3):189-207. 33. Christofidou-Solomidou M, Muzykantov VR. Antioxidant strategies in respiratory medicine. Treat Respir Med. 2006;5(1):47-78. 34. Fukai T, Ushio-Fukai M. Superoxide dismutases: role in redox signaling, vascular function, and diseases. Antioxid Redox Signal. 2011;15(6):1583-606. 35. Jee JP, Lim SJ, Park JS, Kim CK. Stabilization of all-trans retinol by loading lipophilic antioxidants in solid lipid nanoparticles. Eur J Pharm Biopharm. 2006;63(2):134-9. 36. Barros AI, Nunes FM, Goncalves B, Bennett RN, Silva AP. Effect of cooking on total vitamin C contents and antioxidant activity of sweet chestnuts (Castanea sativa Mill.). Food Chem. 2011;128(1):165-72. 37. Tabassum A, Bristow RG, Venkateswaran V. Ingestion of selenium and other antioxidants during prostate cancer radiotherapy: a good thing? Cancer Treat Rev. 2010;36(3):230-4. 38. Waring WS, Webb DJ, Maxwell SR. Systemic uric acid administration increases serum antioxidant capacity in healthy volunteers. J Cardiovasc Pharmacol. 2001;38(3):365-71. 39. He L, He T, Farrar S, Ji L, Liu T, Ma X. Antioxidants Maintain Cellular Redox Homeostasis by Elimination of Reactive Oxygen Species. Cell Physiol Biochem. 2017;44(2):532-53. 40. Paraswani N, Thoh M, Bhilwade HN, Ghosh A. Early antioxidant responses via the concerted activation of NF-kappaB and Nrf2 characterize the gamma-radiation-induced adaptive response in quiescent human peripheral blood mononuclear cells. Mutat Res. 2018;831:50-61.
41. Bravard A, Luccioni C, Moustacchi E, Rigaud O. Contribution of antioxidant enzymes to the adaptive response to ionizing radiation of human lymphoblasts. Int J Radiat Biol. 1999;75(5):639-45. 42. UNSCEAR. UNSCEAR 2013 Report: Sources, effects and risks of ionizing radiation - Volume II Annex B - Effects of radiation exposure of children. 2013. 43. Oenning AC, Jacobs R, Pauwels R, Stratis A, Hedesiu M, Salmon B, et al. Cone-beam CT in paediatric dentistry: DIMITRA project position statement. Pediatr Radiol. 2017. 44. Belmans N, Gilles L, Virag P, Hedesiu M, Salmon B, Baatout S, et al. Method validation to assess in vivo cellular and subcellular changes in buccal mucosa cells and saliva following CBCT examinations. Dentomaxillofac Radiol. 2019.
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45. Macaeva E, Mysara M, De Vos WH, Baatout S, Quintens R. Gene expression-based
biodosimetry for radiological incidents: assessment of dose and time after radiation exposure. Int J Radiat Biol. 2019;95(1):64-75. 46. Pfaffl MW. A new mathematical model for relative quantification in real-time RT-PCR. Nucleic Acids Res. 2001;29(9):e45. 47. Stratis A. Customized Monte Carlo Modelling for Paediatric Patient Dosimetry in Dental and Maxillofacial Cone Beam Computed Tomography Imaging [Doctoral Thesis]. Leuven University Press: KU Leuven; 2018. 48. Stratis A, Zhang G, Lopez-Rendon X, Jacobs R, Bogaerts R, Bosmans H. Customisation of a Monte Carlo Dosimetry Tool for Dental Cone-Beam Ct Systems. Radiation protection dosimetry. 2016;169(1-4):378-85. 49. Stratis A, Touyz N, Zhang GZ, Jacobs R, Bogaerts R, Bosmans H, et al. Development of a paediatric head voxel model database for dosimetric applications. Brit J Radiol. 2017;90(1078). 50. Mitchel RE. Low doses of radiation are protective in vitro and in vivo: evolutionary origins. Dose Response. 2006;4(2):75-90. 51. Eken A, Aydin A, Erdem O, Akay C, Sayal A, Somuncu I. Induced antioxidant activity in hospital staff occupationally exposed to ionizing radiation. Int J Radiat Biol. 2012;88(9):648-53.
Chapter 7: General discussion and future perspectives
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7.1 General discussion
Despite epidemiological evidence about biological risks associated with
exposure to high doses of IR doses, there is no consensus on the risks associated
with low dose IR exposure.(1) However, exposure to low IR doses is highly relevant
to the general public, which is typically exposed to only a few mSv annually. While
about half of this exposure originates from natural sources, the other half is due
to medical diagnostic imaging.(2, 3) Since exposure to IR in medical diagnostics is
the largest man-made source of IR exposure, it is important to know if there are
any health risks associated with it.(2, 4) This is a particular concern in pediatric
patients, since it is known that children are more sensitive to IR than adults.(5, 6)
Although there are epidemiological data available on excessive cancer risk and
childhood exposure to CT or radiography, these studies are performed
retrospectively and have been criticized recently (see Chapter 1). The main aim
of this thesis was to investigate if low doses of X-rays, as those associated with
medical diagnostics, induce DNA damage and oxidative damage, and whether this
is age-dependent. This was examined in vitro in dental stem cells from pediatric
patients and ex vivo in buccal mucosa cells and saliva samples from pediatric and
adult patients undergoing CBCT examinations.
In Chapter 3 and Chapter 4 we describe the ex vivo study that was
conducted in pediatric and adult patients that were subjected to a CBCT
examination. We aimed to investigate if children and adults show similar cellular
and subcellular changes when exposed to very low IR doses.(5, 6) In this context,
the formation of DNA DSBs in BMCs, and oxidative damage and antioxidant status
of saliva samples was studies. In Chapter 3 we describe the optimized study set-
up, as well as the validation of the protocols used in this study. Chapter 4
describes the results from this study.
This study is unique in that sense that saliva samples were used to
investigate the effects of medical imaging (i.e. CBCT) for the first time. Therefore,
prior to the start of patient inclusion, protocols for the ex vivo study were
optimized and validated. In general, blood is the most commonly used sample to
study cellular and subcellular changes after IR exposure. We opted to use BMCs
and saliva since they can be collected in a non-invasive way. Furthermore, it is
cheap and painless.(7-9), which makes this well-suited for pediatric patients. We
validated our buccal swab cell collection method via flow cytometry and bright
field microscopy, and confirmed that over 95% of the cells collected were BMCs.
The use of the γH2AX/53BP1 assay in BMCs was described before.(8, 10-12) Our data
showed that γH2AX/53BP1 can be detected in BMCs collected by buccal swabs.
Furthermore, the saliva collection protocol, based on the passive drool method,
described in chapter 3 allowed us to efficiently collect and store saliva samples
Chapter 7: General discussion and future perspectives
170
from patients. In these saliva samples, we validated the analysis of 8-oxo-
dG/FRAP levels, for two main reasons: 1) to detect 8-oxo-dG/FRAP in saliva
samples, and 2) to investigate if the psychological stress of being subjected to a
medical scan potentially influences the 8-oxo-dG/FRAP levels in saliva. We found
that after actual CBCT examination changes occurred in the 8-oxo-dG and FRAP
levels. But no changes were detected following sham-irradiation. Since sham-
irradiation and the actual CBCT examination occurred in the same patient, we
were confident that the changes in 8-oxo-dG and FRAP levels were due to the
IR.(13)
Despite the validation of our protocols, cautions need to be taken when
using BMCs and saliva samples. BMCs should be collected in a uniform way to
avoid differences in the distribution between cells from the different layers of the
oral mucosa.(7, 14) Our validation experiment showed that our protocol allows for
uniform sampling of BMCs. As with the BM cell distribution, saliva composition can
also be affected by several factors, such as time of collection, the collection
method, intake of dietary supplements, time since last time teeth were brushed,
the presence of blood, etc.. Our collection protocol tries to make collection as
uniform as possible by relying on the passive drool method, which is regarded as
the gold standard.(15) By gathering additional information through questionnaires,
other possible influences can be checked. The protocol that we described can be
used in other settings within radiation protection research. For example, it could
be used in patients subjected to CT examinations, nuclear medicine, or even
interventional radiology. It would also be interesting to apply the protocol to
occupationally exposed populations, such as interventional radiologists. They can
be studied to monitor the response to repeated exposure to low doses of IR.
In chapter 4, we describe the results from these validated tests on BMC and
saliva samples from children and adults. In doing so, we aimed to characterize
the short term radiation-induced effects associated with CBCT examinations,
hereby focussing on potential age-related differences. No DNA DSB induction was
observed in BMCs, neither in children, nor in adults. The γH2AX/53BP1 assay has
been used before to monitor DNA DSB formation after exposure to IR used in
diagnostic and interventional radiology (e.g. CT scans).(16-18) Here it was reported
that the number of DNA DSBs increased following CT examinations in which higher
IR doses are used than in CBCT examinations. Furthermore, in vitro experiments
presented in this thesis have demonstrated that when dental stem cells are
irradiation using a CBCT device, DNA DSBs are induced.(19) γH2AX foci were
detected in BMCs after IR exposure before and our validation experiments
indicated that γH2AX/53BP1 foci could be detected after CBCT examination, we
can assume that CBCT examinations do not statistically significantly increase the
number of DNA DSBs in our patient population.(11, 13) This corresponds to earlier
studies that studied genotoxicity markers following panoramic dental radiography
and CBCT and did not find increased genotoxicity following IR exposure. However,
all these studies reported increases in cytotoxicity markers (e.g. pyknosis and
Chapter 7: General discussion and future perspectives
171
karryorhexis), which was not evaluated in our study (see appendices 2 and 3).
Despite these studies, there are some studies reporting increases in genotoxicity
markers following dental radiography and CBCT examinations (see appendices 2
and 3). Finally, we found that the baseline number of DNA DSBs was statistically
significantly higher in children than in adults. This observation, however, does not
correspond to previous studies that show that aging is associated with an
accumulation of DNA damage, partially due to a reduced DNA repair capacity.(20-
23) Therefore, it was expected that the level of (baseline) DNA damage would be
higher in adults. A possible explanation is that BMCs are the first barrier in
inhalation and ingestion, thus they are exposed to several genotoxins. These
genotoxins can be found in environmental and lifestyle factors such as diet,
mouthwash, smoke, air pollution, etc..(24-26) Children are more sensitive to these
type of genotoxins compared to adults due to age-related differences in
absorption, metabolism, development and body functions.(25) Potentially, this is
the underlying reason as to why the amount of DNA DSBs is statistically
significantly higher in children than in adults.
Next, we observed a significant increase in 8-oxo-dG levels in saliva
samples in children 30 minutes after CBCT examination. In adults, an increase
was also observed, however this increase was not significant. As previously
mentioned, this could be due to a reduced DNA repair capacity in adults. Because
8-oxo-dG has a mutagenic potential, it is removed by the cell when it is sensed
by DNA repair mechanisms (e.g. NER/BER). If these mechanisms operate at
reduced capacity, it could explain why less 8-oxo-dG was detected in saliva in
adults compared to children. Despite this, no significant difference between the
change in 8-oxo-dG excretion was observed between children and adults.
Interestingly, no relation between 8-oxo-dG levels and absorbed dose to the
salivary glands could be observed in our study. What we observe could be similar
to phenomena observed in the ‘adaptive radiation response’. In our case, the IR
doses associated with CBCT result in a small biological response which seems
unrelated to the IR dose, like an all-or-nothing mechanism, similar to the use of
a ‘priming dose’ in adaptive response studies. An adaptive response occurs after
a very low or ‘priming’ dose of a stressor (e.g. a chemical or IR) results in a small
biological response. This small response allows the cell to adapt to the stressor by
activating cellular defence mechanisms against that specific stressor. That way
cells are prepared for an exposure of the same stressor at a higher or ‘challenging’
dose.(27) Our results mimic the effects seen when applying such a ‘priming dose’,
i.e. an effect can be measured, but it is unrelated to the dose of the stressor that
is used. This can be seen in the increase in antioxidant capacity and antioxidant
enzyme activity that we observed in children. These increases were statistically
significant, but were not related with the radiation dose. On the other hand, this
lack of dose response can be due to a high inter-individual variability of radiation
sensitivity. Finally, no gender differences were observed in 8-oxo-dG levels
Chapter 7: General discussion and future perspectives
172
following CBCT examination, neither in children, nor in adults. This resembles
previous studies in urine and other samples of adults.(28-30)
In children we found a significant increase in the total antioxidant capacity
30 minutes after CBCT examination, whereas a significant decrease was found in
adults. These data indicate that children and adults might respond differently to
low doses of IR. Furthermore, besides age-related differences, gender also seems
to play a role in the low dose response. We found that girls, but not boys, showed
a statistically significant increase in FRAP values. Similarly, women, but not men,
displayed a statistically significant decrease in FRAP values. These data indicate
that antioxidant capacity is influenced the most in females, and that the age of
the patient indicates if the antioxidant capacity will increase or decrease. Despite
age- and gender-related differences, no dose-response relationship was seen in
FRAP values, similar to 8-oxo-dG and DNA DSBs levels.
In conclusion, although CBCT induced biological changes, no relationship
with the absorbed radiation dose was observed. This indicates that for low IR
doses, the LNT model does not seem to apply in our patient population.
Furthermore, age at time of exposure seems to correlate to the excretion of 8-
oxo-dG and to the antioxidant response. Furthermore, gender also seems to affect
the antioxidant response. Taken together, these data indicate that even very low
IR doses can elicit biological responses. Therefore, these data should raise
awareness about radiation protection when using CBCT devices. Thus, adherence
to the ALADAIP principle is recommended.(31)
In Chapter 5 we describe the DDR in pediatric dental stem cells in vitro
following low doses of IR. We found that there was a transient induction of DNA
DSBs in SHEDs, DFSCs, and SCAPs. As expected, the number of DSBs was highest
after 30 to 60 minutes and returned to baseline levels 24 hours after radiation
exposure. It is noteworthy that the number of DSBs increased linearly with the
radiation dose in the range of 5 – 100 mGy. Linear regression analysis showed
that 19 – 26 DSBs per Gy were formed, which is in line with observations in
previous studies.(32-36) This analysis also reflected the efficient DNA repair, as the
slope decreases over time until the slope becomes zero after 24 hours, which
indicates that DNA DSBs are effectively repaired. Although these data support a
LNT model, it should be noted that this is an in vitro model and that it does not
give information concerning excessive cancer risk or malignant transformation of
the dental stem cells, which should be considered when applying the LNT model
for risk estimation.(37) No differences were observed in the amount of DSBs
formed, nor in their repair kinetics between the three stem cell types studied here.
Despite the significant induction of DNA DSB that was observed, no major
effects on cell cycle progression were observed. Only in SHEDs, a slight, but
significant, G2/M arrest was seen 72 hours after X-irradiation with 100 mGy. This
was not observed in SCAPs. Although it is known that most cells are most sensitive
to IR in the G2/M phase, our data indicate that radiation sensitivity differs even
Chapter 7: General discussion and future perspectives
173
between similar cell types (SHED vs SCAP). Furthermore, during the G2/M arrest,
DSBs can be repaired both efficiently and error-free through HR.(38) However, DNA
repair kinetics showed that the DSBs were already repaired 24 hours after
irradiation, whereas the G2/M phase arrest was only observed 72 hours after
irradiation. The lack of persistent G2/M arrest can be explained by the fact that
persistent G2/M arrest fails when the number of DSBs are low. It is estimated that
10 – 20 DSBs are required for efficient checkpoint activation.(39, 40) However, it
might be interesting to study the G2/M phase in more detail following low dose IR
exposure, since it is known that there are two distinct types of G2/M checkpoints
that are activated following low IR doses. Both of these checkpoints show cell
type-dependent threshold doses for activation.(41) This could in part explain the
differences between SHEDs and SCAPs that is observed here. Our data indicate
that X-ray doses below 100 mGy, although they cause DNA DSBs, do not cause a
persistent activation of cell cycle checkpoints. This was observed before in
mesenchymal stem cells for both low and high IR doses.(42, 43) However, it would
be interesting to investigate the cell cycle checkpoints at later time point, in order
to see if the G2/M arrest in SHEDs is indeed transient, or that it persists for a
longer period.
Linked to the cell cycle, we report for the first time, a dose-dependent
decrease in the number of G0 phase (quiescent) SHEDs and SCAPs after low dose
X-irradiation. In SCAPs and SHEDs a significant decrease was seen as soon as 1
hour after irradiation. In the latter, this dose-dependent decrease was also
observed 4 hours and 72 hours after irradiation. These data indicate that low X-
ray doses can stimulate dental stem cells to re-enter the cell cycle, which could
lead to a depletion of dental stem cells present. It has been described that an
increase in ROS levels in stem cells could stimulate stem cell proliferation, given
that the ROS concentration are not cytotoxic.(44, 45) We can assume that 100 mGy
of X-rays produces low quantities of ROS, since it is estimated that 1 Gy of γ-rays
(which are similar to X-rays) produces 0.28 µmol•kg-1 OH• and 0.073 µmol•kg-1
H2O2.(46) These quantities could be sufficient to stimulate dental stem cells to re-
enter the cell cycle, without being cytotoxic. It must be noted that we also
observed a time-dependent decrease in the number of G0 phase cells in SHEDs
and SCAPs. This could be due to the build-up of ROS in the culture medium, since
the medium was not changed between irradiation and cell collection. Therefore,
the ROS accumulated this way could also have stimulated the dental stem cells to
reprise the cell cycle, but our data indicates that it is reinforced by low doses of
IR.
Additionally, no premature cellular senescence was observed following low
dose IR exposure in dental stem cells. However, DNA DSBs have been identified
as potent inducers of cellular senescence.(47) Investigation of SASP proteins IL-6,
IL-8, IGFBP-2 and IGFBP-3 indicated a significant time-dependent induction of
senescence but no dose-dependent changes were observed.(48, 49) IL-6 and IL-8
interact with the corresponding surface receptors and, trigger various intracellular
Chapter 7: General discussion and future perspectives
174
signalling cascades. Both are associated with DNA damage-induced premature
senescence. Both can, in a paracrine manner, induce senescence in damaged cells
and their neighbours.(48-50) IGFBP-2 and IGFBP-3 are regulatory factors that
sequester IGF to prevent it binding to its receptor, thereby inhibiting cell
proliferation.(51) Both IGFBP-2 and IGFBP-3 were found to be increased in
senescent cells.(49) Our data indicates that SASP levels of IL-6, IL-8, IGFBP-2 and
IGFBP-3 show similar profiles following X-irradiation. All of them indicate that
there is no dose-dependent increase in the frequency of senescent dental stem
cells following radiation exposure. These data were confirmed by the X-gal assay
showing a time-dependent, but not dose-dependent increase in the frequency of
X-gal positive cells.(52-54) A possible explanation is the lack of persistent DNA
DSBs, which are a potent inducer of senescence.(55) As previously mentioned, no
persistent DSBs were observed in our study. Additionally, it could be that our
methods are not sensitive enough to detect early senescence. It might therefore
be interesting to look at more sensitive assays, such as gene expression markers
or DNA methylation changes.(56-59) Although previous studies showed IR-induced
premature senescence in mesenchymal stem cells, these studies focused on high
doses of IR.(60-63) Radiobiological evidence of low dose IR-induced senescence is
rather scarce.(64, 65) Furthermore, studies that do describe a correlation between
low doses of IR and premature senescence are contradicted by other studies,
including our own.(66, 67)
From our data we can conclude that further research into the biological
consequences of low dose IR exposure (e.g. senescence) on (dental)
mesenchymal stem cells is warranted. Therefore, more in depth radiobiological
studies that focus on more subtle changes should be conducted. Examples are
high-throughput analysis techniques including next-generation sequencing or in-
depth proteomics.
In Chapter 6, preliminary data about the antioxidant response following
CBCT examinations in children and adults is described. The main focus is placed
on anti-oxidant enzymes including SOD1, CAT, and GSH-Px1. These three
important antioxidants were chosen because data from the DIMITRA project (see
Chapter 4) showed that the total antioxidant response differed between children
and adults.(13) Therefore, we assessed antioxidant enzyme activity in saliva
samples. In addition gene expression levels of the three enzymes were studied in
BMCs. Both saliva samples and BMCs were collected in children and adults.
These preliminary data show that, in saliva samples, the SOD and CAT
enzyme activity increases significantly 30 minutes after CBCT examination in
children. A possible explanation is that the enzymatic activity of SOD and CAT
increase in an attempt to scavenge the additional ROS that is formed during the
CBCT examination. Increased enzyme activity is also seen in male inhabitants
(between 50 and 59 years old) of a high background radiation area (5.06 – 6.86
mSv per year) when compared to inhabitants of a control area (i.e. low
Chapter 7: General discussion and future perspectives
175
background radiation; 1.8 – 2.3 mSv per year). Here they found increased SOD,
CAT and GSH-Px1 activities which were probably related to the high background
radiation.(68) Interestingly, in our study the enzyme activity of both SOD and CAT
increased significantly in boys, but not in girls. Similar results were described
before for SOD activity in adult hospital staff. (69) Since the data from DIMITRA
indicate that the total antioxidant response changes with increasing age, it is
possible that this also occurs for the SOD and CAT enzyme activities.
Analysis of gene expression levels for SOD1, CAT, and GPx1 shows that,
overall, the relative gene expression levels decrease after CBCT examination,
though not significantly for CAT. In children, SOD1 gene expression levels
decreased significantly 30 min after CBCT examination and remained decreased
48 hours later. The gene expression levels of GPx1 decreased significantly from
baseline to 48 hours after CBCT examination. These data are not in line with the
enzyme activity assays, but rather seem to indicate a reduced transcription of the
genes of interest. Similar results have been published before, but only after
exposure to high IR doses.(70-72) Furthermore, increases in SOD1, CAT, and GPx1
gene expression levels have been associated with increased radioresistance in
cancer cells.(73-75)
The seemingly contradictory results from the enzyme activity assay and the
gene expression assay indicate that exposure to low IR doses causes several
subtle changes, which are the main reason why it is so difficult to find and validate
good biomarkers for low IR dose exposure. If we compare the results from the
SOD and CAT activity assays with the results from the FRAP assay (Chapter 4),
these data however, support each other. The increase in FRAP values that were
seen in children might be explained by the increased enzyme activity of the SOD
and CAT enzymes that we observed here. However, on the gene level,
contradictory results were obtained since the expression of the genes coding for
these enzymes is reduced. Note that for GPx1 the decrease in gene expression
was only observed after 48 h. At this time point, the enzyme activity was not yet
tested, thus no conclusion can be drawn about the link between gene expression
levels and enzyme activity. Therefore, there is a need for more in-depth research
on the effects (e.g. time-dependency) of low dose exposure on both the genomic
and the proteomic levels.
Our in vitro data indicates that even at low IR doses, between 5 and 100
mGy, the number of DNA DSBs increases linearly with the IR doses. These data
resemble data from high IR dose (i.e. doses over 100 mGy) exposure, and follow
the LNT model (figure 1.12, Chapter 1).(1) However, our patient data indicate that
levels of oxidative damage and the antioxidant response following exposure to low
doses of IR is not correlated with the absorbed dose (10 mGy and below; figure
4.3, Chapter 4). Nevertheless, a measurable response is observed following
exposure to IR during CBCT examinations, both in children and adults. These
responses do not correlate with the different models explaining the dose-response
Chapter 7: General discussion and future perspectives
176
relationship in the low dose range (figure 1.12, Chapter 1). However, since our
data concerns antioxidant responses following exposure to IR, the response we
observed could be linked to a hormetic response. As discussed earlier, exposure
to a low IR dose, could help prepare an organism to an exposure with a higher IR
dose by increasing several defence mechanisms after exposure to the low IR dose,
including antioxidant responses. This priming dose does not necessarily show a
dose response, which could explain our observations.
In conclusion, low IR doses (< 100 mGy) induce significant numbers of DNA
DSBs in dental stem cells in vitro. Despite the increased DNA damage, no effects
on cell cycle progression were observed. However, the number of G0 or quiescent
cells decreases statistically significantly after low dose exposure. This indicates
that the low levels of cellular stress that are caused by the IR stimulate the stem
cells to re-enter the cell cycle. Ex vivo, we did not observe DNA DSBs following
CBCT examination in children nor adults. However, we did find significant
increases in 8-oxo-dG and FRAP levels in children 30 min after CBCT examination.
In adults, a significant decrease in FRAP levels was observed at the same time
point, but no changes in 8-oxo-dG levels were seen. These results indicate that
there is no relation with the IR dose, indicating that the LNT model does not apply
in this low dose range. On the other hand, our results indicate that there is an
age-dependency in the response to IR exposure associated with CBCT
examinations. Furthermore, these data on oxidative stress markers indicate that
both age and gender play a role in the response to low doses of IR associated with
CBCT examinations. Finally, preliminary data on SOD and CAT enzyme activity
indicate that the activity of these important antioxidants increases significantly 30
minutes after CBCT examination in children. This increase was only significant in
boys, not in girls. This observation supports the notion that gender plays a role in
low dose IR response. However, the gene expression levels of the SOD and GPx1
genes indicates that the expression of these genes decreases in BMCs in children
after CBCT examination, and in adults, though only GPx1 expression is decreased
in adults. Therefore, more research is needed to further unravel the complex
biological responses to low dose IR exposure.
Chapter 7: General discussion and future perspectives
177
7.2 Future perspectives
This study demonstrates that low doses of IR usually cause subtle (sub-)
cellular changes. However, in vitro results often do not reflect or predict ex vivo
data.(76-78) For example we showed a linear dose-response in the number of DNA
DSBs following IR exposure in vitro (5 mGy – 100 mGy), but no changes in the
number of DNA DSBs were observed in BMCs ex vivo of patients following CBCT
examination. We admit that during CBCT examinations the IR doses are generally
lower than 5 mGy, but still, one would expect a slight increase based on the in
vitro data. To better understand (sub-)cellular changes following low dose IR
exposure in vitro and ex vivo it might be interesting to include more complex in
vitro models. Furthermore focussing on more specific molecular system and by
using more sensitive detection methods in vitro and ex vivo could increase our
current knowledge.
Organoids, a hot topic in science in recent years, are interesting in vitro
models to study the effects of low dose IR exposure. They reflect the in vivo
environment better than 2D cell cultures. Contrary to 2D cell cultures, 3D
organoids, which are derived from tissue specific stem cells, are miniatures of
selected tissues/organs, and they represent the architecture and even function of
these specific tissues/organs.(79)
The main advantage of organoids is that the effects of low dose IR can be
studied on different cell types of the same tissue, or on different tissues.
Furthermore, this type of 3D in vitro model may help overcome the limitations of
traditional 2D cell culture, such as an overestimation of the IR response.(80-83) For
example, it has been shown that in 2D salivary gland stem cell cultures the
amount of DNA damage is overestimated in comparison with salivary gland
organoids, and that data from organoids better predict the in vivo response in
mice.(81)
However, there are some limitations to the use of organoids. The most
important limitation today is the reproducibility.(84) This can be attributed to the
fact that these organoids do not have the native microenvironment that in vivo
cells have. Therefore it is important to develop co-cultures with immune cells or
other cells to improve the current use of organoids.(80)
To study the effects of CBCT examinations in vitro salivary gland organoids
or oral mucosa organoids could be used.(81, 85-88) These models are highly similar
to the in vivo salivary glands and oral mucosa, respectively. Therefore,
investigating low dose radiation-induced effects in these in vitro models will result
in data which are more indicative for the in vivo situation. This could help us to
reveal potential targets or biomarkers for use in patients who undergo CBCT
examinations.
Chapter 7: General discussion and future perspectives
178
We demonstrated in Chapter 4 that the total antioxidant capacity in saliva
samples changes 30 min after CBCT examination. Furthermore, we found that
these changes are age- and gender-dependent. Therefore, in order to further
understand the low-dose radiation responses, we focused on changes in the
antioxidant enzymes SOD, CAT and GSH-Px1. (see Chapter 6). However, it is
important to look at other antioxidant systems which are present in the cell, such
as the glutathione and thioredoxin systems.(72)
Important members of the glutathione system are: glutathione, glutathione
synthase, glutathione reductase, GSH-Px, and glutathione S-transferase. It has
been suggested that the glutathione system could play a role in counteracting
radiation-induced cerebellar damage.(89) Data from a radiotherapy study has
found that the glutathione levels in serum are depleted after high IR doses (≥ 4
Gy). It was proposed that serum glutathione can be used to predict
chemoradioresponse in cervical cancers.(90, 91) Even after 0.5 Gy exposure in
mouse splenocytes, the glutathione levels increased significantly.(92) Another
study on radiotherapy for brain tumours reported that the levels of glutathione
and the activity of gamma-glutamylcysteine synthetase, which synthesizes
glutathione, are increased after high IR doses.(93) Moreover, one study also
focused on the effect of low-dose irradiation on the glutathione system. In this
study, different responses of the glutathione system to low (1 – 200 mSv) and
high (200 – 1500 mSv) IR doses were measured in children living in the
radionuclide-contaminated regions of Chernobyl.(94) Furthermore, it has been
proposed that glutathione modulates the DNA repair activity, reducing
radiosensitivity.(95)
The thioredoxin system consists of thioredoxin, thioredoxin reductase and
nicotinamide adenine dinucleotide phosphate. The thioredoxin system can also
include peroxiredoxin, which interacts thioredoxin to reduce hydroperoxides and
H2O2.(72, 96) Similarly to the glutathione system, the thioredoxin system plays a
central role in ROS detoxification.(72) In radiotherapy studies, the thioredoxin
system was found to increase its activity after high dose IR exposure, increasing
the radioresistance of tumours.(91) In breast cancer patients, peroxiredoxin levels
were found to predict the clinical outcome following radiotherapy.(97) In
radioresistant lung cancer cells, thioredoxin reductase was identified as
contributing to the radioresistance of these cells.(98) This is also observed in other
cancer cells.(99, 100) After lower IR doses (250 - 1000 mGy) the thioredoxin system
is activated significantly in human blood cells.(101-103)
It is clear that the glutathione and thioredoxin systems are radioresponsive.
Both of them seem to increase following IR exposure, protecting the cells from
oxidative stress and trying to restore the redox balance. However, most studies
were performed in the context of radiotherapy, and thus high IR doses. To the
best of our knowledge, no studies exist that monitor these antioxidant systems
following low dose IR exposure. Therefore, it could be interesting to monitor the
glutathione and thioredoxin systems in patients subjected to medical imaging
Chapter 7: General discussion and future perspectives
179
procedures. This way, more insight can be gathered concerning the antioxidant
response following medical imaging procedures and the maintenance of the redox
balance following low dose IR exposure.
Although organoids and a more in-depth focus on antioxidant systems can
greatly increase the knowledge on low dose IR-induced health effects, more
information could also come from more sensitive techniques, such as liquid-
chromatography tandem mass spectrometry or next-generation sequencing.
These techniques allow for accurate detection of subtle changes on a proteomic
an genetic level, respectively. Therefore, both techniques might be applicable to
unravel inter-individual differences in responses to IR.
Liquid-chromatography tandem mass spectrometry (LC-MS) has a high
specificity and sensitivity. Its multi-analytic potential make it an ideal alternative
to immunoassays or conventional high-performance liquid chromatography.(104)
LC-MS has been used to study exosomes following IR exposure. It was reported
that exosomes from a head and neck cell carcinoma cell line showed changes after
high-dose IR exposure. 236 proteins were detected specifically after irradiation
and 69 proteins were down regulated after irradiation. Proteins overrepresented
in exosomes from irradiated cells were involved in transcription, translation,
protein turnover, cell division and cell signaling, which reflects radiation-induced
changes in cellular processes like transient suppression of transcription and
translation or stress-induced signaling.(105) LC-MS has been used to absolutely
quantify H2AX phosphorylation. Since the formation of γH2AX is an important step
in the DDR, this could be an alternative to immunostaining.(106, 107) LC-MS can also
be used in saliva samples. It was reported that 1256 proteins were identified in
saliva.(108) Since the results presented in Chapters 4 and 6 indicate that low doses
of IR can induce changes in saliva, it is reasonable that with LC-MS more subtle
changes can be detected. Furthermore, it has been described before that the
salivary proteome is radioresponsive.(109) Therefore, it might be an interesting
technique to implement in future low dose IR research in order to identify potential
biomarkers of low dose IR exposure.
Next generation sequencing is a DNA sequencing technique that allows for
the sequencing of the entire human genome in a single day. It also can capture
almost the entire spectrum of mutations that can occur.(110) Next generation
sequencing has revealed that in human fibroblast, there are certain chromosomal
regions that are more prone to accumulating IR-induced alterations than others.
This could point to a characteristic metasignature in the irradiated exome.(111) In
thyroid cancer patients post-Chernobyl, next generation sequencing was used to
detect the underlying genetic alterations underlying the thyroid cancer. Driver
mutations were identified in 96.9% of thyroid cancers, including point mutations
in 26.2% and gene fusions in 70.8% of cases. These data support a link between
thyroid dose and generation of carcinogenic gene fusions associated with radiation
exposure from the Chernobyl accident.(112) Furthermore, it has been shown that
Chapter 7: General discussion and future perspectives
180
the expression of various micro-RNA (miRNA) is altered in IR-exposed cells.
Genome-wide expression changes of miRNA transcriptome by massively parallel
sequencing of human cells exposed to IR, indicated that there are differences in
the expression of many miRNA in a time-dependent fashion following IR exposure.
Six statistically significant temporal expression profiles were identified.(113) This is
important information since it is known that miRNA play an important role in post-
transcriptional gene regulation in X-irradiated cells.(114)
Both LC-MS and next generation sequencing have proven to be powerful
tools. Therefore, they are excellent techniques for research into biomarkers of IR
exposure. In future research projects they can be implemented to look for
biomarkers or to detect changes that are linked to inter-individual variability in
radiosensitivity. To the best of our knowledge, this has not been done in patients
exposed to medical imaging, such as CT or CBCT. Thus the potential findings can
help improve radiation protection guidelines.
Salivary biomarkers have a huge potential when it comes to epidemiological
cohort studies, mostly because it can be collected in a painless, non-invasive
way.(9) Through the use of high-throughput technologies, as described before, the
number of studies describing changes in saliva composition, i.e. salivary
biomarkers, has increased over the last years. Multiple biomarkers for cancer and
non-cancer diseases have been validated in saliva samples, however only a few
salivary biomarkers of IR exposure have been described.(115-117) Thus far, only
three immunomodulatory proteins were described to be linked with full body
irradiations with high IR doses in humans.(109) Data presented in this thesis
indicate that low doses of IR can also induce detectable changes in saliva samples,
namely in 8-oxo-dG concentration. This opens new opportunities to use saliva in
low dose radiation biomarker research.
Despite saliva being more and more used to identify biomarkers of disease,
it is underused for identifying radiation biomarkers. However, due to recent
technological advances, it shows a great potential and should be further
investigated in order to gain more insight into salivary biomarkers of IR exposure.
Saliva samples could, in combination with next-generation sequencing, be
used for genotyping experiments. This way, biomarkers related to genetic variants
could be identified. These biomarkers have potential uses in identifying individual
risks for IR exposure effects and IR susceptibility. Furthermore, saliva samples
could be used to investigate IR-related epigenetic changes. This can be done by
looking at miRNAs in saliva. Since miRNA expression profiles are tissue-specific,
changes due to IR exposure in these profiles could be identified.(118)
As mentioned earlier, LC-MS can also be used in research into salivary
biomarkers. It can provide insight on which proteins respond to both low and high
doses of IR. Furthermore, it can help identify certain metabolites that could be
linked to IR exposure. Unfortunately, the field of metabolomics is still in its
infancy.(119)
Chapter 7: General discussion and future perspectives
181
Currently, radiation protection in medical imaging, both in adults and in
children, is largely based on the ‘as-low-as-reasonable-achievable’ or ALARA
principle. This principle stems from the belief that even if the true cancer risks of
X-ray imaging are not known, minimizing IR exposure was sensible.(120) The
ALARA principle is important for reducing the radiation risk in patients given the
increased use of IR in medical imaging. The ALARA principle mostly focusses on
justification of an examination relying on IR. In short, the benefits should be
weighed against radiation risks, but imaging modalities not utilizing IR such as
ultrasound and magnetic resonance imaging should be considered.(121)
In 2015, the ICRP has published guidelines concerning CBCT. One way to
limit IR dose could be by using the 180°-240° rotation range, instead of a 360°
full rotation. This feature allows keeping radiation-sensitive organs on the detector
side, which results in protection of the more sensitive organs.(122) Other ICRP
recommendations include monitoring of the radiation dose output of the CBCT
device through comparison with reference levels, and using a feedback
mechanisms to the CBCT device leading to automatic adjustment of the X-ray
tube parameters. However, to date, radiation protection emphasises on IR dose
management and avoidance of high dose exposure.(122) In CBCT, the FOV size is
the most significant factor affecting patient dose. Therefore, CBCT devices that
have the option to image small FOVs should be considered. Furthermore, devices
with automatic exposure control are preferred. An example of such the device is
the NewTom VGi EVO that has tube current modulation and which was used in
this study. Other devices used in this study (i.e. Accuitomo and Planmeca) do not
have this option. If manual selection of kV and mA is available, then multiple
choices of kV-mA combinations are recommended to lower exposure settings for
dose optimization.(123) Although these suggestions are well-known, current
(intern)national recommendations for IR dose reduction are inconsistent and too
general. Therefore, the DIMITRA research group aimed at providing indication-
oriented and patient-specific recommendations concerning the use of CBCT in
pediatric patients. This resulted in the newly dubbed ALADAIP principle.(31) This
ALADAIP principle could provide a basis for personalized radiation protection
guidelines, taking into account age, gender, etc., while maintaining adequate
image quality. This personalized radiation protection could be combined with a
radiation passport, allowing radiologists to personalize the radiation dose for each
individual patient. Recently, the DIMITRA research group published results from
a dosimetry study indicating that significant decreases in the effective dose can
be achieved while maintaining the required image quality in pediatric CBCT.(124)
Although reducing the IR dose to which the patient is exposed is one way
to decrease potential radiation induced risks, other measures can be taken to
improve radiation protection. Our data indicates that intracellular antioxidants
increase their activity to defend against the ROS produced by IR. Therefore one
can speculate to use nutritional antioxidants as radioprotective agents. Several
Chapter 7: General discussion and future perspectives
182
studies reported the use of antioxidants as a protective measure against IR-
induced ROS. The general consensus from these studies is that the combination
of several nutritional antioxidants could help relieve the potential harmful effects
of IR exposure through ROS scavenging.(125-128) Other potentially radioprotective
compounds found in food (e.g. garlic, green tea, apples, citrus, and ginger) are
flavonoids, which have antioxidant properties, phenolic acids, and
phytohormones.(129) Today, the use of free radical scavengers is the most common
countermeasure in radioprotection. However, the modulation of growth factors,
cytokines and redox genes are emerging as effective alternative strategies.
Furthermore, gene- and stem cell therapies are being developed as therapeutic
radiation countermeasures and are expected to be applied in the near future to
minimize the side effects of radiation exposure through tissue regeneration.(130)
Although the latter is mostly for exposure to high IR doses, and less relevant for
exposure to low doses such as those used in medical imaging. Finally, it is
noteworthy that there is no conclusive evidence and that the supplementation of
radioprotectors should always be combined with the ALARA and/or ALADAIP
principle.
One important question today in medical imaging is “Dentomaxillofacial
imaging in children, should we be concerned?” Based on our data, CBCT
examinations do not induce DNA DSBs in BMCs in children nor in adults. It does,
on the other hand, induce oxidative damage which was reflected by the significant
increase in 8-oxo-dG levels in saliva samples from children 30 min after CBCT
exposure. Interestingly, this significant increase was not observed in adults.
Furthermore, in saliva samples from children, the total antioxidant capacity
increases significantly 30 minutes after CBCT examination, whereas it decreases
significantly in adults 30 minutes after CBCT examination. These data indicate
that children and adults could respond differently to low doses of IR. Additionally,
30 minutes after CBCT examinations the salivary SOD and CAT activity increase
in children. Gene expression levels of SOD1, and GPx1, however, decrease 30
minutes, and 48 hours after CBCT examination, respectively, in children. In adults
on the other hand, only a decrease in GPx1 gene expression is observed 48 hours
after CBCT examination. These data indicate that in children, antioxidant
responses are activated in order to defend against oxidative stress that is caused
by the CBCT examination. However, at this stage, no conclusion can be made
about the potential long-term effects based on these results. Therefore, it is
recommended to strictly adhere to the ALADAIP principle and to prevent
unnecessary exposure to any form of IR.
Chapter 7: General discussion and future perspectives
183
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Summary
193
One of the greatest challenges in radiation protection today is determining
the detrimental effects of exposure to low doses of ionizing radiation (IR), i.e.
doses lower than 100 mGy. Despite scientific but also public concerns related to
IR doses used in medical imaging, which are well below 100 mGy, the number of
radiological examinations continues to increase. This concern is even more
important in pediatric patients, since it is known that they are more radiosensitive
than adults. Currently some epidemiological data, although controversial, links
computed tomography examination at a young age to increased cancer risk later
in life. However, no such data exists for cone-beam computed tomography
(CBCT).
The aim of this thesis was to investigate if exposure to low doses of X-rays,
more specifically CBCT examinations, induces DNA damage and oxidative
damage. Adults and children were studied to investigate age-related differences.
DNA damage repair kinetics were studied in vitro in dental stem cells and ex vivo
in buccal mucosal cells (BMCs). Oxidative damage was monitored in saliva
samples. Both BMCs and saliva samples were collected from patients before and
after CBCT examination.
After validating the ex vivo set-up (Chapter 3), we conducted a prospective
clinical trial in children and adults who were referred for CBCT examination
(Chapter 4). In both children and adults, no statistically significant induction of
DNA double strand breaks (DSBs) was observed in BMCs gathered 30 minutes and
24 hours after CBCT examination. However, we did observe a significant increase
in the amount of salivary 8-oxo-7,8-dihydro-2’-deoxyguanosine (8-oxo-dG) in
children 30 minutes after CBCT examination, but not in adults. No statistical
difference was observed between children and adults 30 minutes after CBCT
examination. Additionally, a significant increase in salivary total antioxidant
capacity was observed in children, whereas in adults a significant decrease was
seen. This indicates that children and adults might react differently to CBCT
examinations. Interestingly, the observed changes were not linked to the radiation
dose received by the patient.
In vitro exposure to low IR doses of dental stem cells (Chapter 5) causes
a significant increase in DNA DSBs 30 minutes after irradiation. These DSBs are
repaired 24 hours after irradiation. The amount of DSBs increases linearly with
increasing radiation dose in the dose range of 5–100 mGy. This significant
induction of DSBs did not seem to affect the stem cells since no significant cell
cycle changes were observed. However, a significant dose-dependent decrease in
the number of quiescent cells was observed as soon as 1 hour after irradiation.
Furthermore, no premature senescence was induced in dental stem cells following
low dose irradiation.
Finally, preliminary ex vivo data (Chapter 6) indicate that the salivary
activity of the antioxidants superoxide dismutase and catalase increases
significantly 30 minutes after CBCT examination in children. This indicates that
the oxidative damage is countered by endogenous antioxidants. However, when
Summary
194
looking at gene expression level in BMCs, a significant decrease was observed for
GPx1 gene expression 48 hours after CBCT examination in both children and
adults. Furthermore, in children a significant decrease in SOD1 gene expression
was observed 30 minutes and 48 hours after CBCT examination.
In conclusion, our data indicates that tough low doses of IR induce DNA
DSBs in vitro, this does not occur after CBCT examination in BMCs in children and
adults. However, a significant increase in oxidative damage and antioxidant
response was observed in children, but not in adults, suggesting that age does
play a role in the response to low doses of IR. However, no conclusion could be
drawn for long-term effects based on these results. Further research will have to
show if adverse effects occur on the long term. Therefore, it is recommended to
strictly adhere to radiation protection principles and to prevent unnecessary
exposure to any form of IR.
Samenvatting
197
Eén van de grootste uitdagingen in stralingsbescherming vandaag de dag
is het bepalen van negatieve effecten van blootstelling aan lage doses ioniserende
straling, namelijk stralingsdoses lager van 100 mGy. Ondanks bedenkingen van
wetenschappers, maar ook van het grote publiek, omtrent stralingsdoses die
gebruikt worden bij medische beeldvorming, en die ver onder de 100 mGy liggen,
blijft het aantal radiologische onderzoeken toenemen. Deze bezorgdheid is nog
belangrijker als het om kinderen gaat, waarvan geweten is dat zij gevoeliger zijn
dan volwassenen voor de effecten van straling. Momenteel zijn er (controversiële)
epidemiologische data die een verband aantonen tussen computed tomography
scans op jonge leeftijd en een verhoogd kankerrisico op latere leeftijd. Dergelijke
data zijn echter niet voorhanden als het gaat om cone-beam computed
tomography (CBCT).
Het doel van deze thesis was te onderzoeken of blootstelling aan lage
stralingsdoses, zoals gebruikt bij CBCT-scans, DNA-schade en oxidatieve schade
kan veroorzaken. Zowel kinderen als volwassenen werden bestudeerd om
leeftijdsafhankelijke verschillen op te sporen. DNA-schade en de herstelsnelheid
ervan werden in vitro bestudeerd in dentale stamcellen en ex vivo in wangepitheel
cellen (BMCs). Oxidatieve schade werd specifiek onderzocht in speekselstalen.
Zowel BMCs als speekselstalen werden verzameld van patiënten voor en na een
CBCT-scan.
Nadat de ex vivo set-up geoptimaliseerd en gevalideerd werd (Hoofdstuk
3), werd een prospectieve studie uitgevoerd bij kinderen en volwassenen die een
CBCT-scan ondergingen (Hoofdstuk 4). Noch bij kinderen, noch bij volwassenen
werden DNA dubbelstrengsbreuken (DSBs) geobserveerd in BMCs 30 minuten en
24 uur na een CBCT-scan. Er werd echter een significante toename van 8-oxo-
7,8-dihydro-2’-deoxyguanosine geobserveerd in de speekselstalen van kinderen
30 minuten na een CBCT-scan. Bij volwassenen werd dit echter niet
waargenomen. Tussen kinderen en volwassenen werd voor deze merker geen
verschil gevonden 30 minuten na de scan. Wel werd er bij kinderen een
significante stijging waargenomen in de totale antioxidant capaciteit van speeksel,
terwijl er bij volwassenen een significante daling werd vastgesteld. Deze
observatie toont aan dat kinderen en volwassenen verschillend kunnen reageren
op een CBCT-scan. De waargenomen veranderingen vertoonden echter geen
relatie met de stralingsdosis waaraan de patiënt werd blootgesteld.
In vitro blootstelling van dentale stamcellen (Hoofdstuk 5) aan lage
stralingsdoses resulteerde in een significante toename van het aantal DNA DSBs
30 minuten na stralingsblootstelling. De DSBs waren volledig hersteld 24 uur na
stralingsblootstelling. De hoeveelheid DSBs nam lineair toe met de toegediende
stralingsdosis in de range van 5 tot 100 mGy. Deze significante toename van DSBs
lijkt de stamcellen verder niet te beïnvloeden aangezien er geen significante
veranderingen in de celcyclus waargenomen werden. Er werd echter wel een
significante dosisafhankelijke daling van het aantal quiëscente cellen
Samenvatting
198
geobserveerd., dit al vanaf 1 uur na stralingsblootstelling. Verder werd er geen
vervroegde senescentie waargenomen na blootstelling aan lage stralingsdoses.
Ten slotte tonen preliminaire ex vivo data (Hoofdstuk 6) aan dat de
activiteit van de antioxidanten superoxide dismutase en catalase in speekselstalen
toeneemt 30 minuten na een CBCT-scan bij kinderen. Dit wijst er op dat de
oxidatieve schade tegengegaan wordt door endogene antioxidanten.
Genexpressie analyse van deze antioxidanten in BMCs toont aan dat de
genexpressie GPx1 significant daalt 48 uur na een CBCT-scan zowel bij kinderen
als volwassenen. Daarenboven werd bij kinderen ook een significante daling in
SOD1 genexpressie vastgesteld 30 minuten en 48 uur na een CBCT-scan.
We kunnen concluderen dat hoewel lage stralingsdoses DNA DSBs
veroorzaken in dentale stamcellen in vitro, een CBCT-scan geen DSBs veroorzaakt
in BMCs, noch in kinderen, noch in volwassenen. Er vond echter een significante
stijging plaats van oxidatieve schade en van de antioxidant capaciteit in
speekselstalen van kinderen 30 minuten na een CBCT-scan. In volwassenen werd
dit niet waargenomen. Dit suggereert dat leeftijd op moment van blootstelling aan
straling een invloed heeft op de reactie die deze straling veroorzaakt. Momenteel
kan er echter geen conclusie getrokken worden over de lange termijn effecten van
stralingsblootstelling ten gevolge van een CBCT-scan. Verder onderzoek zal
moeten uitwijzen of er al dan niet nadelige effecten optreden op lange termijn.
Daarom wordt er ten zeerste aangeraden om zich te houden aan de principes van
stralingsbescherming, en ook om onnodige stralingsblootstelling zo veel mogelijk
te vermijden.
Appendices
201
Appendix 1: Overview of the biological effects detected in patients following
computed tomography
Assay Gender Age (years)
Gray: Absorbed dose Sievert: Effective dose
Time of sampling
Tissue examined
Tissue used
Biological effects Reference
Dicentric/ring chromosomes
5 patients (gender not specified)
Adults (age not specified)
‘Whole body dose’
NA NA NA Dicentrics and rings significantly increased
Weber et al. (1995)
(1)
5 girls 5 boys
0.4 - 15 Range: 1.2 mGy – 31.3 mGy
Before and 20 min after CT
Thorax (8x) Abdomen (2x)
PBLs*
Dicentrics significantly increased; children younger than 9 are more sensitive than children between 10-15 years old
Stephan et al.
(2007)(2)t
7 females 3 males
62 - 81 Range: 619.1 mGy•cm – 5501.3 mGy•cm
Before CT and 2-28 days after CT
Chest (each patient) Cervix (3x) Abdomen (6x) Pelvis (6x)
Dicentrics significantly increased; no dose Response Abe et al.
(2015)(3)
10 females 17 males
38.3 ± 16.7
Range: 1.18 mGy – 63.36 mGy
Before CT and 2-3h after CT
Abdomen (2x) Thorax (5x) Brain (20x)
Dicentrics significantly increased Kanagaraj
et al. (2015)(4)
15 females 45 males
30 - 83 20.6 ± 9.6 mSv
Before CT and 15 min
Heart (39x) Liver (21x)
Dicentrics and rings significantly increased
Shi et al. (2018)(5)
Appendices
202
and >16h after CT
Micronucleus Assay
10 females 17 males
38.3 ± 16.7
Range: 1.18 mGy – 63.36 mGy
Before CT and 2-3h after CT
Abdomen (2x) Thorax (5x) Brain (20x)
MN frequency significantly increased Kanagaraj
et al. (2015)(4)
13 girls 14 boys
0 – 18 months
Range: 2.2 mGy – 126.1 mGy
2h before and 48h after CT
Abdomen/pelvis (5x) Brain/head (9x) Heart (3x) Chest (12x)
Reticulocytes
No change in MN frequency if there was no prior CT exposure. If there was prior CT exposure, there was a significant increase in MN frequency
Khattab et al.
(2017)(6)
γH2AX assay
23 patients
Adults (age not
specified)
Range: 157 -
1,514 mGy•cm
30 min up to 1 day
after CT
Abdomen Head
(numbers not specified)
PBLs Increased number of γH2AX foci, which was
linearly correlated with the dose-length product
Lobrich et al.
(2005)(7)
8 females 5 males
57 - 74 16.4 mGy (95% confidence interval: 15.1 - 17.7)
Before and 5 to 30 min after CT
Chest (1x) Whole body (12x)
PBMCs** Increased number of γH2AX foci after scan.
Rothkamm et al.
(2007)(8)
5 females (3 with CM***) 22 males (10 with CM)
19 - 84 Range: 10.3 mGy – 13.8 mGy
Before, 0.5h, 1h, 2.5h and 5h after CT
Chest (26x) Chest+Abdomen (1x)
PBLs
Increase in the number of γH2AX foci immediately after CT. Patients examined using iopromide (300 mg of iodine per milliliter) (=CM) show
Grudzenski et al.
(2009)(9)
Appendices
203
30% higher γH2AX foci compared to patients examined without CM.
12 females 22 males
26 - 82 Range: 2.0 mGy – 44.9 mGy
Before and 30 min after CT
Heart (all patients)
Increased number of γH2AX foci and correlation with dose length product.
Kuefner et al.
(2010a)(10)
8 females 28 males
26 - 78 Range: 2.1 mSv – 23.8 mSv
Before and 30 min after CT
Heart (all patients)
Increased number of γH2AX foci and correlation with dose length product.
Kuefner et al.
(2010b)(11)
10 females (4 with CM) 20 males
(11 with CM)
25 - 87 Range: 85 mGy•cm – 900 mGy•cm
Before, 5 min and 1, 2 and 24 h after the CT
Abdomen (all patients)
Increased number of γH2AX foci. γH2AX foci levels were 58% higher in patients undergoing contrast-enhanced CT
(iopromide 370 mg of iodine per milliliter) compared with those undergoing unenhanced CT. After 24h the number of foci returned to baseline levels.
Pathe et al.
(2011)(12)
30 females 39 males
18 - 85 Range: 2.2 mSv – 82.0 mSv
Before and 5 min after contrast-enhanced CT
Vascular (20x) Lungs (16x) Abdomen (21x)
T lymphocytes
Increased number of γH2AX foci. No effect of contrast material (not specified) was observed.
Beels et al. (2012)(13)
19 females 47 males
26 - 82 Range: 1.0 mSv – 23.8 mSv
Before and 30 min after CT angiography
Heart (all patients)
PBLs
Increased number of γH2AX foci and a significant correlation with estimated effective dose was observed.
Brand et al.
(2012)(14)ra
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204
13 females 15 males
60.4 ± 11.0
5.1 mSv ± 2.5 mSv
Before, 1h and 24 h after CT
Heart (all patients)
Increased number of γH2AX foci and excellent correlation between the biological effects and the estimated radiation doses. After 24h the number of foci returned to baseline levels.
Geisel et al.
(2012)(15)
11 females 22 males
29 - 81 Range: 311 – 1751 mGy•cm
Before and at various time points following 18F-Fluorodeoxyglucose application and up to 24 h after CT scan
Whole body (all patients)
Increased number of γH2AX foci and a significant correlation with dose length product was observed.
May et al. (2012)(16)
3 females 4 males
44 - 74 Range: 13.3
mSv – 25.9 mSv
Before and 15 min
after CT
Thorax and/or
abdomen (number not specified)
Increased number of γH2AX foci Kuefner et
al. (2013)(17)
3 boys 0.25 – 1.75
Range: 1.57 mSv – 2.86 mSv
Before and 1h after CT
Not specified
Increased number of γH2AX foci
Halm et al. (2014)(18)
12 females 45 males
56 - 79 Range: 18.8 mSv – 48.8 mSv
Before, 5, 15, 30, 60, and 120 minutes; 6, 24, and 48 hours;
Heart (all patients)
Increased number of γH2AX foci.
Nguyen et al.
(2015)(19)
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205
1 week; and 1 month after CT angiography
149 females (104 with CM) 96 males (75 with CM)
19 - 89 CM: 301 ± 120 mGy•cm No CM: 342 ± 116 mGy•cm
Before and immediately after CT
Chest (all patients)
Increased number of γH2AX foci and dose-enhancing effect of iodine containing contrast material was observed.
Piechowiak et al.
(2015)(20)
14 girls 37 boys
0.1 – 12.2
Range: 0.14 mGy – 2.84 mGy
Before and 5 min after CT
Chest (41x) Abdomen (10x)
T lymphocytes
Increased number of γH2AX foci, exposure to multiple CT scans causes more foci as compared to single scan
Vandevoorde et al. (2015)(21)
5 females 40 males
30 - 76 138.2 ± 62.5 mGy (size-specific dose estimates)
Before, 15 min and a few days after CT Heart (all
patients)
PBLs
Increased number of γH2AX foci and a significant correlation with dose length product was observed.
Fukumoto et al.
(2017)(22)
27 females (15 with CM) 43 males (33 with CM)
29 - 80 CM: 294.3 ± 59.2 mGy•cm No CM: 275.8 ± 40.7 mGy•cm
Before, immediately after CT/CT urography and 8 min after the injection of CM
Urography (48x) Abdomen (22x)
Increased number of γH2AX foci. And dose-enhancing effect of contrast material (33.3 mg of iodine in 90 mL, Ultravist 370) was observed.
Wang et al.
(2017)(23)
Appendices
206
20 females 40 males
17 - 75 Range of averages: 0 – 272.71 mSv
Within 1h after CT
Not specified
Increased number of γH2AX foci was found in cases versus control, the most significant DNA damage amongst cases was observed in cases with multiple CT scans.
Khan et al. (2018)(24)
*: Peripheral blood lymphocytes = PBLs; **: peripheral blood mononuclear cells = PBMCs; ***: contrast medium = CM
1. Weber J, Scheid W, Traut H. Biological dosimetry after extensive diagnostic x-ray exposure. Health Phys. 1995;68(2):266-9. 2. Stephan G, Schneider K, Panzer W, Walsh L, Oestreicher U. Enhanced yield of chromosome aberrations after CT examinations in paediatric patients. Int J Radiat Biol. 2007;83(5):281-7. 3. Abe Y, Miura T, Yoshida MA, Ujiie R, Kurosu Y, Kato N, et al. Increase in dicentric chromosome formation after a single CT scan in adults. Sci Rep. 2015;5:13882. 4. Kanagaraj K, Abdul Syed Basheerudeen S, Tamizh Selvan G, Jose MT, Ozhimuthu A, Panneer Selvam S, et al. Assessment of dose and DNA damages in individuals exposed to low dose and low dose rate ionizing radiations during computed tomography imaging. Mutat Res Genet Toxicol Environ Mutagen. 2015;789-790:1-6. 5. Shi L, Fujioka K, Sakurai-Ozato N, Fukumoto W, Satoh K, Sun J, et al. Chromosomal Abnormalities in Human Lymphocytes after Computed Tomography Scan Procedure. Radiat Res. 2018;190(4):424-32. 6. Khattab M, Walker DM, Albertini RJ, Nicklas JA, Lundblad LKA, Vacek PM, et al. Frequencies of micronucleated reticulocytes, a dosimeter of DNA double strand breaks, in infants receiving computed tomography or cardiac catheterization. Mutat Res-Gen Tox En. 2017;820:8-18. 7. Lobrich M, Rief N, Kuhne M, Heckmann M, Fleckenstein J, Rube C, et al. In vivo formation and repair of DNA double-strand breaks after computed tomography examinations. Proc Natl Acad Sci U S A. 2005;102(25):8984-9. 8. Rothkamm K, Balroop S, Shekhdar J, Fernie P, Goh V. Leukocyte DNA damage after multi-detector row CT: a quantitative biomarker of low-level radiation exposure. Radiology. 2007;242(1):244-51. 9. Grudzenski S, Kuefner MA, Heckmann MB, Uder M, Lobrich M. Contrast medium-enhanced radiation damage caused by CT examinations. Radiology. 2009;253(3):706-14. 10. Kuefner MA, Hinkmann FM, Alibek S, Azoulay S, Anders K, Kalender WA, et al. Reduction of X-ray induced DNA double-strand breaks in blood lymphocytes during coronary CT angiography using high-pitch spiral data acquisition with prospective ECG-triggering. Invest Radiol. 2010;45(4):182-7. 11. Kuefner MA, Grudzenski S, Hamann J, Achenbach S, Lell M, Anders K, et al. Effect of CT scan protocols on x-ray-induced DNA double-strand breaks in blood lymphocytes of patients undergoing coronary CT angiography. European radiology. 2010;20(12):2917-24. 12. Pathe C, Eble K, Schmitz-Beuting D, Keil B, Kaestner B, Voelker M, et al. The presence of iodinated contrast agents amplifies DNA radiation damage in computed tomography. Contrast Media Mol Imaging. 2011;6(6):507-13. 13. Beels L, Bacher K, Smeets P, Verstraete K, Vral A, Thierens H. Dose-length product of scanners correlates with DNA damage in patients undergoing contrast CT. European journal of radiology. 2012;81(7):1495-9.
Appendices
207
14. Brand M, Sommer M, Achenbach S, Anders K, Lell M, Lobrich M, et al. X-ray induced DNA double-strand breaks in coronary CT
angiography: comparison of sequential, low-pitch helical and high-pitch helical data acquisition. European journal of radiology. 2012;81(3):e357-62. 15. Geisel D, Zimmermann E, Rief M, Greupner J, Laule M, Knebel F, et al. DNA double-strand breaks as potential indicators for the biological effects of ionising radiation exposure from cardiac CT and conventional coronary angiography: a randomised, controlled study. European radiology. 2012;22(8):1641-50. 16. May MS, Brand M, Wuest W, Anders K, Kuwert T, Prante O, et al. Induction and repair of DNA double-strand breaks in blood lymphocytes of patients undergoing (1)(8)F-FDG PET/CT examinations. Eur J Nucl Med Mol Imaging. 2012;39(11):1712-9. 17. Kuefner MA, Brand M, Engert C, Kappey H, Uder M, Distel LV. The effect of calyculin A on the dephosphorylation of the histone gamma-H2AX after formation of X-ray-induced DNA double-strand breaks in human blood lymphocytes. Int J Radiat Biol. 2013;89(6):424-32. 18. Halm BM, Franke AA, Lai JF, Turner HC, Brenner DJ, Zohrabian VM, et al. gamma-H2AX foci are increased in lymphocytes in vivo in young children 1 h after very low-dose X-irradiation: a pilot study. Pediatr Radiol. 2014;44(10):1310-7. 19. Nguyen PK, Lee WH, Li YF, Hong WX, Hu S, Chan C, et al. Assessment of the Radiation Effects of Cardiac CT Angiography Using Protein and Genetic Biomarkers. JACC Cardiovasc Imaging. 2015;8(8):873-84. 20. Piechowiak EI, Peter JF, Kleb B, Klose KJ, Heverhagen JT. Intravenous Iodinated Contrast Agents Amplify DNA Radiation Damage at CT. Radiology. 2015;275(3):692-7. 21. Vandevoorde C, Franck C, Bacher K, Breysem L, Smet MH, Ernst C, et al. gamma-H2AX foci as in vivo effect biomarker in children emphasize the importance to minimize x-ray doses in paediatric CT imaging. European radiology. 2015;25(3):800-11. 22. Fukumoto W, Ishida M, Sakai C, Tashiro S, Ishida T, Nakano Y, et al. DNA damage in lymphocytes induced by cardiac CT and comparison with physical exposure parameters. European radiology. 2017;27(4):1660-6. 23. Wang L, Li Q, Wang M, Hao GY, Jie-Bao, Hu S, et al. Enhanced radiation damage caused by iodinated contrast agents during CT examination. European journal of radiology. 2017;92:72-7. 24. Khan K, Tewari S, Awasthi NP, Mishra SP, Agarwal GR, Rastogi M, et al. Flow cytometric detection of gamma-H2AX to evaluate DNA damage by low dose diagnostic irradiation. Med Hypotheses. 2018;115:22-8.
Appendices
209
Appendix 2: Overview of the biological effects detected in patients following X-ray
radiography
Assay Gender Age (years)
Dose Time of sampling
Tissue examined
Tissue used
Biological effects References
Micronucleus assay
24 females 7 males
24 ± 1.023
21.4 µSv
Before and
10 days after examination
Oral cavity
Exfoliated oral mucosa cells
No induction of MN, and cytotoxicity (pyknosis, karyolysis). Significant induction of karyorrhexis.
Cerqueira et al. (2004)(1)
9 girls 8 boys
7.70 ± 1.50
0.08 Roentgen* (Entrance dose)
Angelieri et al. (2007)(2)
42 males 18 - 40 0.057 mSv (Average dose)
Cells of the lateral border of the tongue
No induction of MN, but increased cytotoxicity (pyknosis, karyolysis, karyorrhexis). The number of karyorrhexis and binucleated cells was greater after multiple X-rays
Da Silva et al. (2007)(3)
20 females 12 males
24 - 73 Not mentioned
Before and 10 ± 2 days after examination
Exfoliated oral mucosa cells
No induction of MN, but increased cytotoxicity (pyknosis, karyolysis, karyorrhexis).
Popova et al. (2007)(4)
31 females 9 males
26 ± 9.18 21.4 µSv Before and 10 days after examination
Keratinized gingival cells
Significant induction of MN, and cytotoxicity (pyknosis, karyolysis, karyorrhexis)
Cerqueira et al. (2008)(5)
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210
28 females 11 males
39.6 ± 13 0.08 Roentgen (Entrance dose)
Exfoliated oral mucosa cells
No induction of MN, but increased cytotoxicity (pyknosis, karyolysis, karyorrhexis)
Ribeiro and Angelieri (2008)(6)
6 females 11 males 9 girls 8 boys
39.6 ± 5.4 7.7 ± 1.5
0.08 Roentgen (Entrance dose)
Both in adults and children, no induction of MN, but increased cytotoxicity (pyknosis, karyolysis, karyorrhexis)
Ribeiro et al. (2008)(7)
12 females 20 males
Mean: 38.65
0.08 Roentgen (Entrance dose)
No induction of MN, but increased cytotoxicity (pyknosis, karyolysis, karyorrhexis)
Angelieri et al. (2010a)(8)
12
females 6 males
14.2 ±
1.4
Not
mentioned Angelieri et
al. (2010b)(9)
20 patients (gender not specified)
Children (Age not specified)
Not available
Not mentioned
El-Ashiry et al. (2010)(10)
13 girls 7 boys
4 - 14 Range: 0.13 – 0.29 (entrance dose)
Before and 30 min after examination
Chest Peripheral blood lymphocytes
Significant induction of MN Gajski et al.
(2011)(11)
15 females 15 males
20 - 23 0.046 Roentgen (Entrance dose) Before and
10 days after examination
Oral cavity Exfoliated oral mucosa cells
No induction of MN, but increased cytotoxicity (pyknosis, karyolysis, karyorrhexis)
Ribeiro et al. (2011)(12)
10 females 15 males
11.2 ± 1.4
Not available
Lorenzoni et al. (2012)(13)
80 patients
Adults (age not specified)
Not available
No induction of MN in buccal cells.
Sheikh et al. (2012)(14)
Appendices
211
Significant induction of MN in gingival epithelial cells.
90 patients
Adults (age not specified)
Not available
No induction of MN, but increased cytotoxicity (pyknosis, karyolysis, karyorrhexis)
Thomas et al. (2012)(15)
41 females 19 males
27.63 ± 10.93
0.325 mGy/sec (no exact dose mentioned)
Significant induction of MN Waingade and
Medikeri (2012)(16)
32 females 21 males
25.21 ± 12.67
0.325 mGy/sec (no exact
dose mentioned)
Exfoliated oral mucosa cells and
keratinized gingiva cells
Significant induction of MN in oral mucosa cells and a significant
correlation was observed between the age of the subjects and number of MN
Arora et al. (2014)(17)
20 patients (gender not specified)
Children (age not specified)
21.4 mSv (average dose)
Exfoliated oral mucosa cells
No induction of MN, but increased cytotoxicity (pyknosis, karyolysis, karyorrhexis)
Agarwal et al. (2015)(18)
20 girls 20 boys
7 - 12 Not mentioned
Before and 10 ± 2 days after examination
Significant induction of MN Preethi et al.
(2016)(19)
70 females 28 males
23.63 ± 6.64
Range: 0.18 mGy – 3.54 mGy
Before and 10 days after examination
Significant induction of MN, and cytotoxicity (pyknosis, karyolysis, karyorrhexis) above 1 mGy. Below 1 mGy, only significant
Li et al. (2018)(20)
Appendices
212
induction of karyorrhexis.
Comet assay
14 girls 6 boys
5 - 14 Range: 0 – 0.29
Before and 30 min after examination
Chest Peripheral blood lymphocytes
Significant increase of DNA damage following radiography.
Milkovic et al. (2009)(21)
20 patients (gender not specified)
Adults (age not specified)
Not mentioned
Before and 30 min or 24h after examination
Oral cavity
Exfoliated oral mucosa cells
Significant increase of DNA damage 30 min following radiography, but not after 24h
Yanuaryska et al. (2018)(22)
γH2AX assay
45 females 55 males
20 - 77 23.4 mGy (average dose)
Before and 20 min after examination
Oral cavity Exfoliated oral mucosa cells
Increased number of γH2AX foci.
Yoon et al. (2009)(23)
20 females
39 - 71 Range: 7.1 – 41.1
Before and 5 min after examination
Breasts Systemic blood lymphocytes
Schwab et al. (2013)(24)
*: 1 Roentgen (R) = 2.58 x 10-4 C/kg
1. Cerqueira EM, Gomes-Filho IS, Trindade S, Lopes MA, Passos JS, Machado-Santelli GM. Genetic damage in exfoliated cells from oral mucosa of individuals exposed to X-rays during panoramic dental radiographies. Mutat Res. 2004;562(1-2):111-7. 2. Angelieri F, de Oliveira GR, Sannomiya EK, Ribeiro DA. DNA damage and cellular death in oral mucosa cells of children who have undergone panoramic dental radiography. Pediatr Radiol. 2007;37(6):561-5. 3. da Silva AE, Rados PV, da Silva Lauxen I, Gedoz L, Villarinho EA, Fontanella V. Nuclear changes in tongue epithelial cells following panoramic radiography. Mutat Res. 2007;632(1-2):121-5. 4. Popova L, Kishkilova D, Hadjidekova VB, Hristova RP, Atanasova P, Hadjidekova VV, et al. Micronucleus test in buccal epithelium cells from patients subjected to panoramic radiography. Dentomaxillofac Radiol. 2007;36(3):168-71. 5. Cerqueira EM, Meireles JR, Lopes MA, Junqueira VC, Gomes-Filho IS, Trindade S, et al. Genotoxic effects of X-rays on keratinized mucosa cells during panoramic dental radiography. Dentomaxillofac Radiol. 2008;37(7):398-403. 6. Ribeiro DA, Angelieri F. Cytogenetic biomonitoring of oral mucosa cells from adults exposed to dental X-rays. Radiat Med. 2008;26(6):325-30. 7. Ribeiro DA, de Oliveira G, de Castro G, Angelieri F. Cytogenetic biomonitoring in patients exposed to dental X-rays: comparison between adults and children. Dentomaxillofac Radiol. 2008;37(7):404-7. 8. Angelieri F, de Cassia Goncalves Moleirinho T, Carlin V, Oshima CT, Ribeiro DA. Biomonitoring of oral epithelial cells in smokers and non-smokers submitted to panoramic X-ray: comparison between buccal mucosa and lateral border of the tongue. Clin Oral Investig. 2010;14(6):669-74.
Appendices
213
9. Angelieri F, Carlin V, Saez DM, Pozzi R, Ribeiro DA. Mutagenicity and cytotoxicity assessment in patients undergoing orthodontic
radiographs. Dentomaxillofac Radiol. 2010;39(7):437-40. 10. El-Ashiry EA, Abo-Hager EA, Gawish AS. Genotoxic effects of dental panoramic radiograph in children. J Clin Pediatr Dent. 2010;35(1):69-74. 11. Gajski G, Milkovic D, Ranogajec-Komor M, Miljanic S, Garaj-Vrhovac V. Application of dosimetry systems and cytogenetic status of the child population exposed to diagnostic X-rays by use of the cytokinesis-block micronucleus cytome assay. J Appl Toxicol. 2011;31(7):608-17. 12. Ribeiro DA, Sannomiya EK, Pozzi R, Miranda SR, Angelieri F. Cellular death but not genetic damage in oral mucosa cells after exposure to digital lateral radiography. Clin Oral Investig. 2011;15(3):357-60. 13. Lorenzoni DC, Cuzzuol Fracalossi AC, Carlin V, Araki Ribeiro D, Sant' Anna EF. Cytogenetic biomonitoring in children submitting to a complete set of radiographs for orthodontic planning. Angle Orthod. 2012;82(4):585-90. 14. Sheikh S, Pallagatti S, Grewal H, Kalucha A, Kaur H. Genotoxicity of digital panoramic radiography on oral epithelial tissues. Quintessence Int. 2012;43(8):719-25. 15. Thomas P, Ramani P, Premkumar P, Natesan A, Sherlin HJ, Chandrasekar T. Micronuclei and other nuclear anomalies in buccal mucosa following exposure to X-ray radiation. Anal Quant Cytol Histol. 2012;34(3):161-9. 16. Waingade M, Medikeri RS. Analysis of micronuclei in buccal epithelial cells in patients subjected to panoramic radiography. Indian J Dent Res. 2012;23(5):574-8. 17. Arora P, Devi P, Wazir SS. Evaluation of genotoxicity in patients subjected to panoramic radiography by micronucleus assay on epithelial cells of the oral mucosa. J Dent (Tehran). 2014;11(1):47-55. 18. Agarwal P, Vinuth DP, Haranal S, Thippanna CK, Naresh N, Moger G. Genotoxic and cytotoxic effects of X-ray on buccal epithelial cells following panoramic radiography: A pediatric study. J Cytol. 2015;32(2):102-6. 19. Preethi N, Chikkanarasaiah N, Bethur SS. Genotoxic effects of X-rays in buccal mucosal cells in children subjected to dental radiographs. BDJ Open. 2016;2:16001. 20. Li G, Yang P, Hao S, Hu W, Liang C, Zou BS, et al. Buccal mucosa cell damage in individuals following dental X-ray examinations. Sci Rep. 2018;8(1):2509. 21. Milkovic D, Garaj-Vrhovac V, Ranogajec-Komor M, Miljanic S, Gajski G, Knezevic Z, et al. Primary DNA damage assessed with the comet assay and comparison to the absorbed dose of diagnostic X-rays in children. Int J Toxicol. 2009;28(5):405-16. 22. Yanuaryska RD. Comet Assay Assessment of DNA Damage in Buccal Mucosa Cells Exposed to X-Rays via Panoramic Radiography. J Dent Indones. 2018;25(1):53-7. 23. Yoon AJ, Shen J, Wu HC, Angelopoulos C, Singer SR, Chen R, et al. Expression of activated checkpoint kinase 2 and histone 2AX in exfoliative oral cells after exposure to ionizing radiation. Radiat Res. 2009;171(6):771-5. 24. Schwab SA, Brand M, Schlude IK, Wuest W, Meier-Meitinger M, Distel L, et al. X-ray induced formation of gamma-H2AX foci after full-field digital mammography and digital breast-tomosynthesis. PLoS One. 2013;8(7):e70660.
Curriculum Vitae
215
Appendix 3: Overview of the biological effects detected in patients following cone
beam computed tomography
Assay Gender Age (years)
Dose Time of sampling
Tissue examined
Tissue used
Biological effects References
Micronucleus (MN) assay
9 females 10 males
26.8 ± 5.0
Not mentioned
Before and 10 days after cone beam computed tomography
Oral cavity
Exfoliated oral mucosa cells
No induction of MN, but induction cytotoxicity (pyknosis, karyolysis, karyorrhexis)
Carlin et al. (2010)(1)
10 girls 14 boys
11 ± 1.2 Range: 287 µSv - 304 µSv
Lorenzoni et al. (2013)(2)
39 females 7 males
23 - 42 Range: 448.15 - 730.79 mGy·cm2
Yang et al. (2017)(3)
17 females 12 males
45.8 ± 12.5
Not mentioned
Significant induction of MN, and cytotoxicity (pyknosis, karyolysis, karyorrhexis)
Da Fonte et al. (2018)(4)
70 females 28 males
23.63 ± 6.64
Range: 0.18 mGy – 3.54 mGy
Significant induction of MN, and cytotoxicity (pyknosis, karyolysis, karyorrhexis) above 1 mGy. Below 1 mGy, only significant induction of karyorrhexis.
Li et al. (2018)(5)
Curriculum Vitae
216
1. Carlin V, Artioli AJ, Matsumoto MA, Filho HN, Borgo E, Oshima CT, et al. Biomonitoring of DNA damage and cytotoxicity in individuals
exposed to cone beam computed tomography. Dentomaxillofac Radiol. 2010;39(5):295-9. 2. Lorenzoni DC, Fracalossi AC, Carlin V, Ribeiro DA, Sant'anna EF. Mutagenicity and cytotoxicity in patients submitted to ionizing radiation. Angle Orthod. 2013;83(1):104-9. 3. Yang P, Hao S, Gong X, Li G. Cytogenetic biomonitoring in individuals exposed to cone beam CT: comparison among exfoliated buccal mucosa cells, cells of tongue and epithelial gingival cells. Dentomaxillofac Radiol. 2017;46(5):20160413. 4. da Fonte JBM, de Andrade TM, Albuquerque RLC, de Melo MDB, Takeshita WM. Evidence of genotoxicity and cytotoxicity of X-rays in the oral mucosa epithelium of adults subjected to cone beam CT. Dentomaxillofac Rad. 2018;47(2). 5. Li G, Yang P, Hao S, Hu W, Liang C, Zou BS, et al. Buccal mucosa cell damage in individuals following dental X-ray examinations. Sci Rep. 2018;8(1):2509.
List of publications
219
Personal information
Surname: Belmans
First name: Niels
Address: Putstraat 1 0102, 2470 Retie, Belgium
Mobile phone: +32 472 71 81 04
Email: niels.belmans@gmail.com
Date of birth: April 19th, 1992
Place of birth: 2400 Mol, Belgium
Nationality: Belgian
Civil class: Unmarried
Education
2015 – Present PhD student, Biomedical Sciences
University of Hasselt, Morphology Group, Biomedical Research
Institute, Hasselt, Belgium;
Belgian Nuclear Research Centre (SCK•CEN), Mol, Belgium
Director: Prof. Ivo Lambrichts (UHasselt)
Co-director: Prof. Stéphane Lucas (UNamur) & Dr. Marjan
Moreels (SCK•CEN)
Thesis title: Biological effects of ionizing radiation in medical
imaging: a prospective study in children and adults following
cone-beam computed tomography
2013 – 2015 Master in Biomedical Sciences (Great distinction)
University of Antwerp, Department of Biomedical Sciences,
Antwerp, Belgium
Major: Clinical Scientific Research
Minor: Entrepreneurship and research
Thesis director: Prof. Sylvia Dewilde
Thesis title: Endothelial cell response after exposure to low
dose X-ray radiation
Laboratory Animal course: FELASA C obtained
2010 – 2013 Bachelor in Biomedical Sciences (Great distinction)
University of Antwerp, Department of Biomedical Sciences,
Antwerp, Belgium
Thesis director: Prof. Xaveer Van Ostade
Thesis title: Molecular mechanisms for Withaferine A.
2004 – 2010 Latin-Sciences
Rozenberg, S.O., Mol, Belgium
List of publications
220
Travel grants and Awards
2018 Research Award (oral presentation)
Awarded at the European Congress of Dentomaxillofacial Radiology held
in Luzern, Switzerland.
2017 ERRS Young Investigator Award
Travel support to attend the ERRS Annual Meeting in Essen, Germany.
2017 EU CONCERT Travel Grant
Grant to attend the 4th International Symposium on the System of
Radiological Protection of ICRP and for the 2nd European Radiation
Protection Research Week of the European Research Platforms in Paris,
France.
Professional memberships
2018 – Present Netherlands Society for Radiobiology (NVRB)
2018 European Academy for Dentomaxillofacial Radiology (EADMFR)
2016 – Present European Radiation Research Society (ERRS)
2015 – Present Belgian Society for the Advancement in Cytometry (BSAC)
Courses attended
2018 Grow yourself leadership course – UHasselt, Belgium
2018 FLAMES: GDPR – UHasselt, Belgium
2017 Career management in academia – UHasselt, Belgium
2017 Good clinical practices course – UHasselt, Belgium
2017 Good laboratory practices course – UHasselt, Belgium
2017 Basic biosafety training – UHasselt, Belgium
2016 FLAMES: Tools for time series – KU Leuven, Belgium
2016 Effective image editing – UHasselt, Belgium
2016 Effective graphical displays – UHasselt, Belgium
2016 PhD management: Successfully applying project & time
management principles – UHasselt, Belgium
2016 Self-, peer-, and co-assessment course – UHasselt, Belgium
2015 Good Scientific Conduct course – UHasselt, Belgium
2015 Scientific Writing and Speaking – SCK•CEN, Belgium
2015 Upgrade your written English – SCK•CEN, Belgium
2015 FLAMES: Significance, p-values and t-tests – UGhent, Belgium
2015 Basic training in Radiation Protection – SCK•CEN, Belgium
List of publications
221
Supervision of students
10/2016 – 06/2017 Liese Gilles (MSc), Biomedical Sciences, UHasselt,
Hasselt, Belgium
09/2017 – 01/2018 Kristof Smeets (BSc),Biotechnology, PXL, Hasselt,
Belgium
02/2018 – 06/2018: Jonas Welkenhuysen (BSc), Biotechnology, PXL,
Hasselt, Belgium
02/2019 – 06/2019: Jasper Gielen (BSc), Biotechnology, PXL, Hasselt,
Belgium
List of publications
225
Publications in peer-reviewed journals
Published
1 Baselet B, Belmans N, Coninx E, Lowe D, Janssen A, Michaux A, Tabury K,
Raj K, Quintens R, Benotmane MA, Baatout S, Sonveaux P and Aerts A
(2017) Functional Gene Analysis Reveals Cell Cycle Changes and
Inflammation in Endothelial Cells Irradiated with a Single X-ray Dose. Front.
Pharmacol. 8:213. doi: 10.3389/fphar.2017.00213
2 Virag P, Hedesiu M, Soritau O, Perde-Schrepler M, Brie I, Pall E, Fischer-
Fodor E, Bogdan L, Lucaciu O, Belmans N, Moreels M, Salmon B, Jacobs R
Low-dose radiations derived from cone beam computed tomography induce
transient DNA and inflammatory alterations in stem cells from deciduous
teeth. DMFR (2018) 47. doi: 10.1259/dmfr.20170462
3 Oenning AC, Pauwels R, Stratis A, De Faria Vasconcelos K, Tijskens E, De
Grauwe A, Jacobs R, Salmon B, Chaussain C, Bosmans H, Bogaerts R, Politis
C, Nicolielo L, Zhang G, Vranckx M, Ockerman A, Baatout S, Belmans N,
Moreels M, Hedesiu M, Virag P, Baciut M, Marcu M, Almasan O, Roman R,
Barbur I, Dinu C, Rotaru H, Hurubeanu L, Istouan V, Lucaciu O, Leucuta D,
Crisan B, Bogdan L, Candea C, Bran S, Baciut G; Halve the dose while
maintaining image quality in paediatric cone beam CT – Sci Rep (2019)
9:5521 doi: 10.1038/s41598-019-41949-w
4 Belmans N, Gilles L, Virag P, Hedesiu M, Salmon B, Baatout S, Lucas S,
Jacobs R, Lambrichts I, Moreels M; Method validation to assess in vivo
cellular and subcellular changes in buccal mucosa cells and saliva following
CBCT examinations – DMFR (2019) 48. doi: 10.1259/dmfr.20180428
5 Konings K, Vandevoorde C, Belmans N, Vermeesen R, Baselet B, Van
Walleghem M, Janssen A, Isebaert S, Baatout S, Haustermans K, Moreels
M; The combination of particle irradiation with the Hedgehog inhibitor
GANT61 differently modulates migration of cancer cells compared to X-ray
irradiation. Front. Oncol. (2019) 9:391 doi: 10.3389/fonc.2019.00391
Submitted
Belmans N, Gilles L, Vermeesen R, Virag P, Hedesiu M, Salmon B, Baatout S,
Lucas S, Lambrichts I, Jacobs R, Moreels M; Dental cone-beam CT examination
induces oxidative damage and antioxidant response in children’s saliva – Nature
Scientific Reports – In review
List of publications
226
In preparation
1. Belmans N, Gilles L, Vermeesen R, Virag P, Hedesiu M, Salmon B, Baatout
S, Lucas S, Lambrichts I, Jacobs R, Moreels M – Increased oxidative damage
and antioxidant response in saliva samples from children following cone
beam computed tomography examination - in vivo results paper
2. Belmans N, Gilles L, Salmon B, Baatout S, Lucas S, Lambrichts I, Moreels
M – In vitro assessment of the DNA damage response in dental stem cells
following low dose X-ray exposure – in vitro results paper
3. Belmans N, Baatout S, Moreels M – Health risks following medical X-ray
diagnostics: Should we be concerned? – Literature review
Oral presentations
1. Belmans N, Baatout S, Moreels M; Tandheelkundige röntgenfoto’s bij
kinderen: Moeten we ons zorgen maken?; EHS Instituutsvergadering,
February 19th, 2019, Mol, Belgium
2. Belmans N, Moreels M, Baatout S.; Biological effects of ionizing radiation
in medical imaging: A prospective study in children and adults following
dental cone-beam CT; Dutch Society for Radiobiology (NVRB), November
16th, 2018, Utrecht, The Netherlands
3. Belmans N, Moreels M, Baatout S, Lambrichts I; Increased oxidative stress
and an adaptive antioxidant response in saliva after dental CBCT exposure
in children; 44th European Radiation Research Congress, August 24th, 2018,
Pecs, Hungary
4. Belmans N., Baatout S., Moreels M.; Dental CBCT exposure in children:
can we detect biological changes in saliva samples?; European Congress of
Dentomaxillofacial Radiology, June 14th, 2018, Luzern, Switzerland
5. Belmans N; Dental CBCT exposure in children: can we detect biological
changes in saliva samples?; Day of the PhDs, April 24th, 2018, Mol, Belgium
6. Belmans N, Gilles L, Vranckx M, Baatout S, Jacobs R, Lucas S, Lambrichts
I, Moreels M; Age-related biological effects of dental cone-beam CT
exposure; 2nd European Radiation Protection Research Week of the
European Research Platforms, October 11th, 2017, Paris, France
7. Belmans N, Gilles L, Lucas S, Lambrichts I, Moreels M; Age-related
biological effects of dental cone-beam CT exposure; 43rd European
Radiation Research Congress, September 18th, 2017, Essen, Germany
8. Belmans N; DIMITRA Subtask 1: Characterizing the potential risks through
radiation biology; Final OPERRA Meeting, May 24th, 2017, Budapest,
Hungary
9. Belmans N; DIMITRA Task 1: In vitro DNA damage response and ex vivo
DNA damage and oxidative stress analysis after dental CBCT examination
List of publications
227
in children and adults; DIMITRA Team Meeting, January 13th, 2017, Cluj-
Napoca, Romania
10. Belmans N; Dental pediatric imaging: an investigation towards low dose
radiation induced risks; Day of the PhDs, October 27th, 2016, Mol, Belgium
11. Belmans N; Dental CBCT in children: In vitro and ex vivo DNA damage and
oxidative stress analysis; Belgian Society for the Advancement of
Cytometry Annual Meeting, October 21st, 2016, Brussels, Belgium
12. Belmans N, Moreels M, Baatout S; Impact of dental cone-beam CT in
children: Low dose radiation effects on dental stem cells, buccal cells and
saliva; OPERRA 2nd Periodic Meeting, June 9th, 2016, Kuopio, Finland
13. Belmans N, Baatout S, Lambrichts I, Moreels M; Dental pediatric imaging:
an investigation into low dose radiation-induced risks, Seminar at BIOMED,
February 29th, 2016, Hasselt, Belgium
14. Belmans N, Baatout S, Lambrichts I, Moreels M; Dental pediatric imaging:
an investigation into low dose radiation-induced risks, EHS Meet & Greet,
January 18th, 2016, Mol, Belgium
15. Belmans N, Baatout S, Lambrichts I, Moreels M; Dental pediatric imaging:
an investigation into low dose radiation-induced risks – Protocols and
necessities, DIMITRA Team Meeting, December 10th, 2015, Leuven,
Belgium
List of publications
228
Poster presentations
1. Belmans N, Vermeesen R, Baselet B, Moreels M; Saliva: Potential use as a
health marker on Earth and in Space?; 25 years of Belgians in Space Event,
October 6th, 2017, Mol, Belgium
2. Belmans N, Gilles L, Lambrichts I, Moreels M; Age-related biological effects
of dental cone-beam CT exposure; Knowledge for Growth, May 18th, 2017,
Ghent, Belgium
3. Belmans N; Dental CBCT in children: In vitro and ex vivo DNA damage and
oxidative stress analysis; WAC Audit, October 26th, 2016, Mol, Belgium
4. Belmans N, Moreels M, Baatout S, Stratis A, Tijskens E, Bosmans H,
Bogaerts R, Lambrichts I, Salmon B, Baciut M, Hedesiu M, Virag P, Jacobs
R; DIMITRA Task 1: Assessing biological risks: Optimization of buccal swab
and saliva collection protocols for pilot study in children; OPERRA 2nd Annual
Meeting, June 7th-9th, 2016, Kuopio, Finland
5. Piroska Virag, Mihaela Hedesiu, Salmon Benjamin, Niels Belmans, Lucaciu
Ondine, Mihaela Baciut, Reinhilde Jacobs. - Low dose radiation induced
effects in dental pulp stem cells; OPERRA 2nd periodic meeting, June 7th-
9th 2016, Kuopio, Finland
6. R. Jacobs, M. Hedesiu, M.Baciut, B. Salmon, A. Stratis, H. Bosmans, R.
Bogaerts, C. Chaussain, S. Baatout, H. Derradji, N. Belmans, M. Moreels,
A. Michaux, J. Buset, R Roman, M Marcu, V. Piroska, I. Barbur, H.Rotaru,
C.Dinu, O.Almasan, D.Leucuta, B. Crisan, L. Bogdan, A. Coman, Gr. Baciut.
- DIMITRA project Task 3: Epidemiology: cumulative radiation exposure and
risk from dento-maxillofacial radiology during childhood.; OPERRA 2nd
periodic meeting, June 7th-9th, 2016, Kuopio, Finland
7. R. Jacobs, H. Bosmans, R. Bogaerts, A. Stratis, C. Chaussain, B. Salmon,
A. Oenning, M. Cohen, S. Baatout, H. Derradji, N. Belmans, M. Moreels,
A. Michaux, J. Buset, M. Baciut, M. Hedesiu, V. Piroska. - DIMITRA Task 4
- Reducing risks through image quality optimization; OPERRA 2nd periodic
meeting, June 7th-9th, 2016, Kuopio, Finland
8. R. Jacobs, H. Bosmans, R. Bogaerts, E. Van de Casteele, A. Stratis, C.
Chaussain, B. Salmon, D. Le Denmat, S. Baatout, H. Derradji, N. Belmans,
M. Moreels, A. Michaux, J. Buset, M. Baciut, M. Hedesiu, V. Piroska. -
DIMITRA: Dentomaxillofacial paediatric imaging: an investigation towards
low dose radiation induced risks. – MELODI 7th workshop; November 9th-
11th, 2015, Munich, Germany
Acknowledgements
231
Equipped with his five senses, man explores the universe around him and calls
the adventure 'science'.
Edwin Powell Hubble
My personal adventure started almost four years ago. And as with all adventures,
a PhD project comes with ups and (unfortunately) downs. Luckily, I have met a
lot of wonderful people along the way that helped me achieve the highs, but also
(and maybe more importantly) guided me through the lows. In the end, all of
them helped me to evolve as a human being and scientist. Since this PhD thesis
would not have been possible without these individuals, I would like to devote this
section to all who have contributed to my four-year-long adventure.
First of all, I would like to thank Prof. Dr. Ivo Lambrichts, my promotor. Thank
you for accepting me as a PhD student. I really appreciate your critical and
valuable comments and feedback on my work throughout the PhD project. I could
not thank you enough for sharing your knowledge and expertise. Prof. Dr.
Stéphane Lucas, my co-promotor, I would also like to express my gratitude for
your support and feedback on my work. Prof. Dr. Sarah Baatout, thank you for
welcoming me into the Radiobiology Unit at SCK•CEN. Your continuous support
and passion for science, and radiobiology in particular, are inspiring. Dr. Marjan
Moreels, my SCK•CEN mentor and co-promotor, I thank you for your amazing
support and guidance over the last four years. Thank you for our nice discussions,
your (many) critical revisions of my work, and helping me out whenever I had
questions or doubts. You were always there when I needed advice or just a friendly
chat. Thank you!
I also like to thank Prof. Dr. Annelies Bronckaers for being part of my internal
doctoral committee. Thank you for the time you took for reviewing my work and
for your valuable and thorough evaluation of my work. Furthermore, I would like
to thank Prof. Dr. Reinhilde Jacobs and Prof. Dr. Benjamin Salmon for their
willingness to be my external jury members, for reviewing my PhD thesis and for
their constructive feedback. For chairing my doctoral jury, I would like to thank
Prof. Dr. Marcel Ameloot.
I am also indebted to the members of DIMITRA. Once again my thanks goes out
to Prof. Dr. Reinhilde Jacobs, fearless leader of the DIMITRA team. Your drive
and enthusiasm are unmatched (of this I am sure!). Furthermore, I owe thanks
to Prof. Dr Benjamin Salmon for helping (and providing me) with the dental
Acknowledgements
232
stem cells that were a crucial part of this project. Your advice and guidance
concerning the cell cultures and data analyses are much appreciated. Next I would
like to thank dr. Piroska Virag and dr. Mihaela Hedesiu. Thank you for
reviewing my papers, but also for the nice team meetings and the unsurpassed
hospitality you showed us when we visited Cluj-Napoca! For help with patient
dosimetry, thanks is due to dr. Andreas Stratis and dr. Ruben Pauwels.
Thanks to your Monte Carlo simulations we obtained valuable data for our
analyses. Thank you dr. Anne Carolina Costa Oenning, dr. Karla de Faria
Vasconcelos, dr. Jeroen van Dessel, dr. Raluca Roman, Myrthel Vranckx,
Anna Ockerman and Bennaree Awarun for your support and friendship, and
the wonderful time at ECDMFR2018!
Of course Myrthel Vranckx and Anna Ockerman, along with Gabriela
Casteels, Jeroen Martens and Birgit Coucke, deserve additional thanks for
helping me with collecting patient samples (kudos to all St.-Raphael staff that also
contributed!), keeping patient records and navigating the biobank legislation. This
thesis could not be completed without your help and support!
I would like to thank EHS scientists dr. Pieter Monsieurs, dr. Mohamed Mysara
and dr. Jürgen Claesen for their help and support with my questions regarding
statistics and experimental set-ups.
From the Laboratory for Nuclear Calibrations of SCK•CEN, I would like to thank
Bart Marlein, Raf Aarts, and dr. Cristian Mihailescu for their help with setting
up and performing the in vitro irradiations. Thank you Bart and Raf for the nice
chats during the long waiting periods during the many irradiations.
Special thanks to Betty Vandingelen and Veronique Pousset, secretaries at
SCK•CEN and UHasselt, respectively. Thank you for all your help with all the paper
work and for all the nice chats.
I owe thanks to Ann Janssen, Amelie Coolkens, Kevin Tabury, and (most
importantly, since he is probably the best lab technician EVER ) Randy
Vermeesen for their help with cell cultures, microscopy, flow cytometry and
protein assays. Especially Kevin Tabury, who taught me most of what he knows
about microscopy at the beginning of my PhD, and Randy Vermeesen, who
helped me with pretty much everything I asked him in the last part of my PhD,
deserve my gratitude. Thank you Randy for being a great colleague and friend.
It was my pleasure to help you with Luminex experiments in the morning, with
freeze drying, …
I could not have coped without my office buddies. Thank you Anu Yadav, Gleb
Goussarov, and especially dr. Bo Byloos. Anu we started our PhD together and
shared all the hard times and struggles that come with doing a PhD. Gleb, you
Acknowledgements
233
started your PhD adventure two years ago and you still have a long road ahead,
but you’ll undoubtedly make it! And Bo, like you said before ‘we hemme da toch
mer wee schoun gedon!’. I will always remember our (weird) talks and discussions
(does ‘slechte vlesekes’ ring a bell?), our mutual love for cinema, and of course,
our 2016 road trip a.k.a. ‘The Pimped Out Adventures of Bno and Neil’, which was
AMAZING! To me, as an office buddy you will always be second to none!
Dr Marlies Gijs, dr. Annelies Suetens, dr. Tine Verreet, dr. Ellina Macaeva,
dr. Kai Craenen, and dr. Katrien Konings, the PhDs that came before, thank
you for showing me that it can be done and for all the advice and fun moments in
the lab. Special thanks to Katrien, who gave me the opportunity to participate in
irradiation campaigns at GANIL. Thank you for the nice experience, fun and stress
in the lab, but most importantly, thank you for showing me the importance of
accurately planning experiments ahead of time .
Dr. Bjorn Baselet, you deserve your own paragraph. You were an amazing
mentor during my master thesis. You taught me much (if not all) of what I knew
about lab work at that time. You are also responsible for the fact I had to write
this beast. Your enthusiasm and confidence in my abilities convinced me to apply
for a PhD position way back in 2014. And look where we are now! Through the
years you have become a great friend, and I will cherish all the fun moments we
have had in the lab and during PhD dinners, movie nights, after-work events, …
Monsieur (!) Claude Mfossa, Raghda Ramadan, Ali Muntasir, Valérie Van
Eesbeeck, Emma Coninx, Noami Daems, Auchi Inalegwu, Eline Radstake,
Magy Sallam, Charlotte Segers, Shari Wouters, Laurens Maertens, Tom
Rogiers, and Merel Van Walleghem, the other PhD students at SCK•CEN, I
thank you for the wonderful times both in and outside the lab. I wish you all the
best for what is to come.
I would also like to thank Liese Gilles, Kristof Smeets, and Jonas
Welkenhuysen. Your contributions can be found all throughout this thesis.
Without your help, I could not have accomplished the massive feat that is/was
this PhD project. Furthermore, it was my great pleasure to guide you in your
MSc/BSc theses. Special thanks is due to Natalie Alderson and Isatou Sheriff,
who voluntarily (!) helped me out during the summer holidays. Thank you all for
your assistance and all the fun we had in the lab. Kudos to all other students that
were also responsible for the fun and great atmosphere in the lab (you know who
you are ).
Acknowledgements
234
Ik wil uiteraard ook mijn vrienden en vriendinnen bedanken voor hun steun en de
nodige afleidingen in de afgelopen vier jaar. Door jullie kon ik de nodige stoom
aflaten en op tijd en stond mijn zinnen verzetten.
Daarnaast wil ik ook mijn familie bedanken. Eerst en vooral mijn ouders. Ik wil
jullie bedanken voor jullie onvoorwaardelijke steun tijdens dit doctoraat en ook
voor alles daarbuiten. Ik kan niet beschrijven hoeveel jullie hulp en steun voor mij
betekenen. Ook Jolien, mijn jongere zusje, wil ik bedanken. Hoewel we soms
bekvechten en nu al een tijdje niet meer in het ouderlijk huis wonen, weet ik dat
ik altijd op jou kan rekenen als ik hulp nodig heb. Mijn grootouders wil ik ook
bedanken voor hun onvoorwaardelijke steun. Va en moemoe, waar jullie
ondertussen ook zijn, ik heb jullie steun op elk moment gevoeld.
Als laatste wil ik Liese bedanken. Woorden schieten te kort om te beschrijven
hoeveel ik aan jou te danken heb. Jij kwam (terug?) in mijn leven halverwege
mijn doctoraat. Sindsdien heb je mijn leven op ontelbare manieren positief
beïnvloedt. Jouw liefde en steun gaven me de nodige energie om dit doctoraat
succesvol af te ronden. Weet dat ik dit zonder jou niet had gekund!
P.S. For those who requested a copy of my thesis solely to check if they are
mentioned in the ‘Acknowledgements’ section (I’m looking at you Randy), I
strongly suggest you take some time to read the rest. There is some pretty
interesting stuff in there
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